Installation of Embedded MQTT Broker with 2.5.0-SNAPSHOT fails AND/OR connection refused

this is your issue. Please follow steps in linked thread for workaround. The issue is now solved but the change did not make it in time for M4 release. Any of the snapshots shortly after M4 should contain the fix

link to git hub issue
https://github.com/openhab/openhab2-addons/issues/5527

more info