Seems like an error in your or your ISP’s configuration. I know you said nothing changed, so maybe your ISP. The original error in this thread (caused by Roon server outage, expected to be resolved since November 14) resulted in no error message, the Roon ARC settings continued to say “ready”, and no error.
Your error looks exactly like one I solved just recently here, also with an Asus, an RT-AX82U in this case:
However, in this case the issue was an ISP router in front of the Asus router, which necessitated two port forwarding rules, one on the ISP router and one on the Asus router. You don’t seem to have this according to your description, and anyway it worked previously for you, so I guess it’s something different.
Nevertheless, probably something specific to you and not a general Roon server problem. It’s timing out and not getting an external IP address (“router_external_ip”:“null”)
Another similar error here, but also caused by having two routers: