Tradfri - Communication error

Hi, I am trying to get the Tradfri binding running, but it always ends in a communication error. I am sure that the IP and the security code are correct (pinged IP, checked against MAC and c&p security code from QR code).

The only issue I can think of is that the OpenHAB server is in a different subnet. Routing between the two works, though.
Any other thoughts?

Yes, read this and provide the information that we need to help you, please.

OpenHAB version: 2.3.0-1
Tradfri binding fresh installed from PaperUI, configured via PaperUI.
openjdk version “1.8.0_191”
Ubuntu 18.10


10:09:01.869 [INFO ] [smarthome.event.ThingUpdatedEvent    ] - Thing 'tradfri:gateway:f17c36b1' has been updated.
10:09:01.874 [WARN ] [tradfri.handler.TradfriGatewayHandler] - Gateway with old firmware - please consider upgrading to the latest version.
10:09:02.029 [INFO ] [smarthome.event.ThingUpdatedEvent    ] - Thing 'tradfri:gateway:f17c36b1' has been updated.
10:09:02.122 [INFO ] [ome.event.ThingStatusInfoChangedEvent] - 'tradfri:gateway:f17c36b1' changed from OFFLINE (COMMUNICATION_ERROR) to UNKNOWN

Hm, looking at other forums, people also have issues connecting via VPN. Some probably the gateway rejects connects from other subnets. Pitty. OpenHAB stays in the DMZ.