Hi @Mark_Worsey,
You’re unfortunately encountering a known issue - if ARC worked fine for months and then displayed a blank diagnostic, then you’re in the group affected by this bug: Unable to Access Roon ARC Outside Home Network After Recent IPv6 Compatibility Update [Fix in Early Access]
Fortunately, we’re testing a fix right now in Early Access. If you move your RoonServer machine and ARC over to this #earlyaccess branch (here are quick instructions), you’ll likely be able to run ARC. This fix will go into the public build in the next release once we’ve verified efficacy.
If ARC had previously worked and you encounter a blank diagnostic screen without an IP address in Roon → Settings → ARC, then please see the above.