The issue is the same described here Courier IMAP (4.15) stopped synchronizing moves after ~~Play Store~~ F-Droid(?) updated me to K9-12.1 and the workaround in the linked github issue is working for me (disable automatic IMAP namespace).
The issue is the same described here Courier IMAP (4.15) stopped synchronizing moves after ~~Play Store~~ F-Droid(?) updated me to K9-12.1 and the workaround in the linked github issue is working for me (disable automatic IMAP namespace).