Hi @blechkiste,
Thank you for your patience while we’ve worked through the queue of port forwarding tickets.
We’ve recently released a fix for an issue with Roon 2.0 affecting port assignments (release notes here). While there’s certainly a possibility that the culprit for your issue resides elsewhere in the network infrastructure, it could be that the port listed (in the 35000-36000 range) was unavailable, due to a port range assignment issue in Roon.
I recommend updating both Roon and ARC to the latest build, released today, and then attempting the automatically configure port forwarding again. Should you continue to experience the same or a similar set of symptoms, please provide the diagnostic message (obscuring any routable IP address) as you did in your original post. We should be equipped to help you more efficiently and pin down the issue, or at least provide a suitable workaround.
Thank you again for your patience.