I have the exact same problem. My remotes (phone & tablet) have been automatically updated, but the NUC-based ROCK core fails to update. Rebooted it twice, reinstalled the OS once; internet connection fully functional, but the core either keeps “checking for an update” or reports a failure.
My macOS laptop Roon-app has not been updated and happily connects to the core.
EDIT: OK, obviously not the exact same issue here. Reboots didn’t solve the issue for me.
Symptoms are the same though.