This started happening to me about four days ago as well. All was fine, I was able to play music and now my Roon Core cannot be found.
I can access my NUC’s ‘system status’ page and it tells me the OS, server and DB are all fine. Reinstalling the OS, restarting the sever software and rebooting my NUC make no difference.
My system is Roon Core running on a NUC which is hardwired to my Netgear Orbi. I have a Pi endpoint which is also hardwired to my router. All other devices and applications on this network are working fine.
It was while a song was playing that the dropout occurred and since then I cannot reconnect.