This generally means that the Roon Server is using uPnP to request that the router forward a port that is already being forwarded to another device.
The solution is usually to change the port used by ARC. In your case, it is currently set to 55002. Try setting it to 56000 or even 60000.
The port number on ethernet networks is a 16 bit value and so is limited to the range 0 to 65535. In addition, port numbers below 1024 are reserved for known servers (e.g. 80 and 8080 for HTTP, 443 for HTTPS etc) and so these should be avoided.
Your router may have a status page showing you all of the port forwarding rules that are current - both those explicitely created and those configured by uPnP. This page should show you what ports are free (in the 1024 to 65535 region). Pick a port that is not used by any of these forwarding rules and you should be good to go.