Follow-up to previous post: Please also try fully exiting Opus (via File / Exit Directory Opus) to see if it starts working again after a restart. (Don't just close the window, as it will still be running in the background. Use the explicit Exit option, please.)
I think I just reproduced the problem, but it went away again after I did this, and I'm now no longer able to reproduce it again. There does seem to be a problem here somewhere, but triggering it seems to need something specific that we need to nail down in order to understand what's going wrong and fix it.