I can retrace to have rebooted at 22:55.
I was in the terminal, not observing the tail output.
… which may have interfered with OH3 while is was still starting?!
I had a look at the link, and given all seemed dead, I issued the follwoing commands:
sudo openhab-cli stop
sudo openhab-cli clean-cache
# [2022-07-26 21:56] openhabian@openhabian ~ $
sudo systemctl start openhab.service
The resulting log file shows multiple self-restarts of OH… (not sure, whether this is expected behaviour)
2022-07-26 21:55:29.730 [INFO ] [org.openhab.ui.internal.UIService ] - Stopped UI
==> /var/log/openhab/events.log <==
==> /var/log/openhab/openhab.log <==
2022-07-26 22:00:37.455 [INFO ] [org.openhab.ui.internal.UIService ] - Started UI on port 8080
2022-07-26 22:01:46.391 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Time zone set to 'Australia/Brisbane'.
2022-07-26 22:01:46.421 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Locale set to 'en_AU'.
2022-07-26 22:01:46.425 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Measurement system set to 'SI'.
2022-07-26 22:02:12.988 [INFO ] [e.automation.internal.RuleEngineImpl] - Rule engine started.
2022-07-26 22:02:21.660 [INFO ] [.core.model.lsp.internal.ModelServer] - Started Language Server Protocol (LSP) service on port 5007
2022-07-26 22:07:12.341 [INFO ] [org.openhab.ui.internal.UIService ] - Stopped UI
2022-07-26 22:07:27.787 [ERROR] [core.karaf.internal.FeatureInstaller] - Failed installing 'openhab-binding-mqtt, openhab-persistence-rrd4j, openhab-ui-basic, openhab-binding-avmfritz, openhab-binding-http, openhab-binding-darksky, openhab-ui-habpanel, openhab-binding-systeminfo, openhab-binding-zigbee': null
2022-07-26 22:08:21.539 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Time zone set to 'Australia/Brisbane'.
2022-07-26 22:08:21.576 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Locale set to 'en_AU'.
2022-07-26 22:08:21.579 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Measurement system set to 'SI'.
2022-07-26 22:08:22.987 [INFO ] [org.openhab.ui.internal.UIService ] - Started UI on port 8080
2022-07-26 22:08:25.026 [WARN ] [.transport.servlet.ServletController] - Can't find the request for http://192.168.1.68:8080/rest/auth/token's Observer
2022-07-26 22:08:25.051 [WARN ] [.transport.servlet.ServletController] - Can't find the request for http://192.168.1.68:8080/rest/auth/logout's Observer
2022-07-26 22:08:25.115 [WARN ] [.transport.servlet.ServletController] - Can't find the request for http://192.168.1.68:8080/rest/'s Observer
2022-07-26 22:08:45.246 [INFO ] [.core.model.lsp.internal.ModelServer] - Started Language Server Protocol (LSP) service on port 5007
2022-07-26 22:08:47.726 [INFO ] [e.automation.internal.RuleEngineImpl] - Rule engine started.
==> /var/log/openhab/events.log <==
2022-07-26 22:10:49.009 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:broker:c60c2e942a' changed from UNINITIALIZED to INITIALIZING
2022-07-26 22:10:49.163 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:broker:c60c2e942a' changed from INITIALIZING to OFFLINE
2022-07-26 22:10:49.601 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:coordinator_ember:42b78e1deb' changed from UNINITIALIZED to INITIALIZING
2022-07-26 22:10:50.014 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:device:42b78e1deb:a4c138de80ed67f1' changed from UNINITIALIZED to UNINITIALIZED (BRIDGE_UNINITIALIZED)
==> /var/log/openhab/openhab.log <==
2022-07-26 22:10:54.035 [WARN ] [core.thing.internal.ThingManagerImpl] - Initializing handler for thing 'mqtt:broker:c60c2e942a' takes more than 5000ms.
2022-07-26 22:11:00.727 [INFO ] [bee.discovery.ZigBeeDiscoveryService] - A4C138DE80ED67F1: Starting ZigBee device discovery
2022-07-26 22:11:01.537 [WARN ] [uth.internal.AbstractAuthPageServlet] - Authentication failed from 192.168.1.13: CSRF check failed
==> /var/log/openhab/events.log <==
2022-07-26 22:11:01.152 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:coordinator_ember:42b78e1deb' changed from INITIALIZING to ONLINE
2022-07-26 22:11:01.187 [INFO ] [penhab.event.FirmwareStatusInfoEvent] - Firmware status of thing zigbee:coordinator_ember:42b78e1deb changed to UNKNOWN.
==> /var/log/openhab/openhab.log <==
2022-07-26 22:11:02.861 [INFO ] [bee.discovery.ZigBeeDiscoveryService] - A4C138DE80ED67F1: Starting ZigBee device discovery
==> /var/log/openhab/events.log <==
2022-07-26 22:11:07.022 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:device:42b78e1deb:a4c138de80ed67f1' changed from UNINITIALIZED (BRIDGE_UNINITIALIZED) to INITIALIZING
2022-07-26 22:11:07.040 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:device:42b78e1deb:a4c138de80ed67f1' changed from INITIALIZING to UNKNOWN
2022-07-26 22:11:07.169 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'avmfritz:fritzbox:192_168_1_1' changed from UNINITIALIZED to INITIALIZING
2022-07-26 22:11:07.210 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'avmfritz:fritzbox:192_168_1_1' changed from INITIALIZING to UNKNOWN
2022-07-26 22:11:07.306 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'FRITZbox7490_IncomingCall' changed from NULL to UNDEF
==> /var/log/openhab/openhab.log <==
2022-07-26 22:11:09.870 [INFO ] [.transport.mqtt.MqttBrokerConnection] - Starting MQTT broker connection to '192.168.1.5' with clientid 61f4d268-1ad6-4577-bb74-c89887ed753b
==> /var/log/openhab/events.log <==
2022-07-26 22:11:10.053 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:device:42b78e1deb:a4c138de80ed67f1' changed from UNKNOWN to ONLINE
2022-07-26 22:11:10.136 [INFO ] [penhab.event.FirmwareStatusInfoEvent] - Firmware status of thing zigbee:device:42b78e1deb:a4c138de80ed67f1 changed to UNKNOWN.
2022-07-26 22:11:10.168 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'Shed_KDL_Colour' changed from NULL to 0,0,0
2022-07-26 22:11:10.172 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'Shed_KDL_Dimmer' changed from NULL to 0
2022-07-26 22:11:10.254 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'avmfritz:fritzbox:192_168_1_1' changed from UNKNOWN to ONLINE
2022-07-26 22:11:10.565 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:broker:c60c2e942a' changed from OFFLINE to ONLINE
==> /var/log/openhab/openhab.log <==
2022-07-26 22:11:29.106 [INFO ] [ab.ui.habpanel.internal.HABPanelTile] - Started HABPanel at /habpanel
2022-07-26 22:11:29.228 [WARN ] [ty.util.ssl.SslContextFactory.config] - Trusting all certificates configured for Client@d61ba5[provider=null,keyStore=null,trustStore=null]
2022-07-26 22:11:29.231 [WARN ] [ty.util.ssl.SslContextFactory.config] - No Client EndPointIdentificationAlgorithm configured for Client@d61ba5[provider=null,keyStore=null,trustStore=null]
==> /var/log/openhab/events.log <==
2022-07-26 22:11:50.193 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:topic:baca0c37ec' changed from UNINITIALIZED to INITIALIZING
2022-07-26 22:11:50.312 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:topic:baca0c37ec' changed from INITIALIZING to ONLINE
==> /var/log/openhab/openhab.log <==
2022-07-26 22:12:28.867 [WARN ] [uth.internal.AbstractAuthPageServlet] - Authentication failed from 192.168.1.13: CSRF check failed
==> /var/log/openhab/events.log <==
2022-07-26 22:12:59.560 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:topic:baca0c37ec' changed from ONLINE to UNINITIALIZED
2022-07-26 22:12:59.626 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:topic:baca0c37ec' changed from UNINITIALIZED to UNINITIALIZED (HANDLER_MISSING_ERROR)
==> /var/log/openhab/openhab.log <==
2022-07-26 22:13:00.047 [INFO ] [org.openhab.ui.internal.UIService ] - Stopped UI
==> /var/log/openhab/events.log <==
2022-07-26 22:13:04.393 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'avmfritz:fritzbox:192_168_1_1' changed from ONLINE to UNINITIALIZED
2022-07-26 22:13:04.424 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'avmfritz:fritzbox:192_168_1_1' changed from UNINITIALIZED to UNINITIALIZED (HANDLER_MISSING_ERROR)
2022-07-26 22:13:04.448 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:device:42b78e1deb:a4c138de80ed67f1' changed from ONLINE to UNINITIALIZED
2022-07-26 22:13:04.477 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:device:42b78e1deb:a4c138de80ed67f1' changed from UNINITIALIZED to UNINITIALIZED (HANDLER_MISSING_ERROR)
2022-07-26 22:13:04.512 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:broker:c60c2e942a' changed from ONLINE to UNINITIALIZED
2022-07-26 22:13:04.690 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:broker:c60c2e942a' changed from UNINITIALIZED to OFFLINE (COMMUNICATION_ERROR): Client sent DISCONNECT
2022-07-26 22:13:05.300 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:broker:c60c2e942a' changed from OFFLINE (COMMUNICATION_ERROR): Client sent DISCONNECT to OFFLINE
2022-07-26 22:13:05.408 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:broker:c60c2e942a' changed from OFFLINE to UNINITIALIZED (HANDLER_MISSING_ERROR)
2022-07-26 22:13:05.435 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:coordinator_ember:42b78e1deb' changed from ONLINE to UNINITIALIZED
2022-07-26 22:13:08.554 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:coordinator_ember:42b78e1deb' changed from UNINITIALIZED to UNINITIALIZED (HANDLER_MISSING_ERROR)
==> /var/log/openhab/openhab.log <==
2022-07-26 22:13:12.105 [INFO ] [ab.ui.habpanel.internal.HABPanelTile] - Stopped HABPanel
2022-07-26 22:13:12.176 [INFO ] [basic.internal.servlet.WebAppServlet] - Stopped Basic UI
2022-07-26 22:13:36.981 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Time zone set to 'Australia/Brisbane'.
2022-07-26 22:13:37.014 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Locale set to 'en_AU'.
2022-07-26 22:13:37.017 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Measurement system set to 'SI'.
2022-07-26 22:13:58.898 [INFO ] [.core.model.lsp.internal.ModelServer] - Started Language Server Protocol (LSP) service on port 5007
2022-07-26 22:14:07.074 [INFO ] [org.openhab.ui.internal.UIService ] - Started UI on port 8080
2022-07-26 22:14:07.979 [INFO ] [e.automation.internal.RuleEngineImpl] - Rule engine started.
2022-07-26 22:15:33.221 [INFO ] [org.openhab.ui.internal.UIService ] - Stopped UI
2022-07-26 22:15:47.866 [ERROR] [core.karaf.internal.FeatureInstaller] - Failed installing 'openhab-binding-mqtt, openhab-persistence-rrd4j, openhab-ui-basic, openhab-binding-avmfritz, openhab-binding-http, openhab-binding-darksky, openhab-ui-habpanel, openhab-binding-systeminfo, openhab-binding-zigbee': null
2022-07-26 22:15:59.937 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Time zone set to 'Australia/Brisbane'.
2022-07-26 22:15:59.972 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Locale set to 'en_AU'.
2022-07-26 22:15:59.975 [INFO ] [.core.internal.i18n.I18nProviderImpl] - Measurement system set to 'SI'.
2022-07-26 22:16:01.145 [INFO ] [org.openhab.ui.internal.UIService ] - Started UI on port 8080
2022-07-26 22:16:19.720 [INFO ] [.core.model.lsp.internal.ModelServer] - Started Language Server Protocol (LSP) service on port 5007
2022-07-26 22:16:21.056 [INFO ] [e.automation.internal.RuleEngineImpl] - Rule engine started.
==> /var/log/openhab/events.log <==
2022-07-26 22:16:38.804 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:device:42b78e1deb:a4c138de80ed67f1' changed from UNINITIALIZED to UNINITIALIZED (BRIDGE_UNINITIALIZED)
2022-07-26 22:16:39.136 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:coordinator_ember:42b78e1deb' changed from UNINITIALIZED to INITIALIZING
2022-07-26 22:16:43.796 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'avmfritz:fritzbox:192_168_1_1' changed from UNINITIALIZED to INITIALIZING
2022-07-26 22:16:43.828 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'avmfritz:fritzbox:192_168_1_1' changed from INITIALIZING to UNKNOWN
2022-07-26 22:16:43.905 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'FRITZbox7490_IncomingCall' changed from NULL to UNDEF
2022-07-26 22:16:45.538 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'avmfritz:fritzbox:192_168_1_1' changed from UNKNOWN to ONLINE
==> /var/log/openhab/openhab.log <==
2022-07-26 22:16:46.233 [INFO ] [bee.discovery.ZigBeeDiscoveryService] - A4C138DE80ED67F1: Starting ZigBee device discovery
==> /var/log/openhab/events.log <==
2022-07-26 22:16:46.413 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:coordinator_ember:42b78e1deb' changed from INITIALIZING to ONLINE
2022-07-26 22:16:46.422 [INFO ] [penhab.event.FirmwareStatusInfoEvent] - Firmware status of thing zigbee:coordinator_ember:42b78e1deb changed to UNKNOWN.
==> /var/log/openhab/openhab.log <==
2022-07-26 22:16:46.958 [INFO ] [bee.discovery.ZigBeeDiscoveryService] - A4C138DE80ED67F1: Starting ZigBee device discovery
==> /var/log/openhab/events.log <==
2022-07-26 22:16:51.439 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:device:42b78e1deb:a4c138de80ed67f1' changed from UNINITIALIZED (BRIDGE_UNINITIALIZED) to INITIALIZING
2022-07-26 22:16:51.459 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:device:42b78e1deb:a4c138de80ed67f1' changed from INITIALIZING to UNKNOWN
2022-07-26 22:16:51.773 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:broker:c60c2e942a' changed from UNINITIALIZED to INITIALIZING
==> /var/log/openhab/openhab.log <==
2022-07-26 22:16:51.926 [WARN ] [ty.util.ssl.SslContextFactory.config] - Trusting all certificates configured for Client@232485[provider=null,keyStore=null,trustStore=null]
2022-07-26 22:16:51.929 [WARN ] [ty.util.ssl.SslContextFactory.config] - No Client EndPointIdentificationAlgorithm configured for Client@232485[provider=null,keyStore=null,trustStore=null]
==> /var/log/openhab/events.log <==
2022-07-26 22:16:51.932 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:broker:c60c2e942a' changed from INITIALIZING to OFFLINE
2022-07-26 22:16:52.284 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'Shed_KDL_Colour' changed from NULL to 0,0,0
2022-07-26 22:16:52.498 [INFO ] [openhab.event.ItemStateChangedEvent ] - Item 'Shed_KDL_Dimmer' changed from NULL to 0
2022-07-26 22:16:52.699 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'zigbee:device:42b78e1deb:a4c138de80ed67f1' changed from UNKNOWN to ONLINE
2022-07-26 22:16:52.703 [INFO ] [penhab.event.FirmwareStatusInfoEvent] - Firmware status of thing zigbee:device:42b78e1deb:a4c138de80ed67f1 changed to UNKNOWN.
==> /var/log/openhab/openhab.log <==
2022-07-26 22:16:54.139 [INFO ] [.transport.mqtt.MqttBrokerConnection] - Starting MQTT broker connection to '192.168.1.5' with clientid 1db9871c-b188-49fa-9339-2cc9ff8395f0
==> /var/log/openhab/events.log <==
2022-07-26 22:16:54.504 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:broker:c60c2e942a' changed from OFFLINE to ONLINE
==> /var/log/openhab/openhab.log <==
2022-07-26 22:17:05.627 [INFO ] [ab.ui.habpanel.internal.HABPanelTile] - Started HABPanel at /habpanel
==> /var/log/openhab/events.log <==
2022-07-26 22:17:05.959 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:topic:baca0c37ec' changed from UNINITIALIZED to INITIALIZING
2022-07-26 22:17:06.225 [INFO ] [ab.event.ThingStatusInfoChangedEvent] - Thing 'mqtt:topic:baca0c37ec' changed from INITIALIZING to ONLINE
Ending up stable 20 minutes later.
I could login and all that was before is still there. 
So far so good.
While I am happy to see this post as resolved, I am still curious why I have multiple restarts in a row, and it took 20 minutes. The latter could be related to the cache deletion as Udo indicated in the quoted link.