This looks like a configuration problem in your network for ARC, not the temporary issue on the Roon side that was the cause of this thread, i.e.:
Most likely, you have UPnP enabled on the router, and you have a conflicting manual port forwarding rule configured on the router. Fixing the router configuration (either disable UPnP on the router or delete the manual rule) may fix this. Else, it will be best if you open a new topic for this error in ARC: Port Forwarding Help