For the past few weeks or so, DEVONthink Pro Office > Quit DEVONthink Pro Office is grayed out / disabled. The only way I can now quit the app is to force quit it.
I am using the latest version (2.9.11) on macOS Sierra. I double checked, and this happens even when no windows are open. I went to Tools > Verify & Repair, and it didn’t find anything serious. Even so, I had it Repair the few minor errors. Even after that, the Quit command remains grayed out. It’s not even set to sync any databases currently, so a scheduled sync is not what’s keeping it open.
I searched the board, and found that in past years, some people reported similar problems, and at times, it was reported to be a known behavior that was in the process of being fixed. I didn’t find anything recent, though.