I’m having the same problem now. Roon Core is on a streamer, Salkstreamer III running Linux. The error shows up on all 3 of my Roon controllers; iPad mini 2, iPhone 6s, and desktop computer running Windows 10.
It seems to have started since the last update, 1.2 build 165.
It will usually start working again on its own if I wait awhile (20 minutes maybe?).