[Dovecot] FETCH for mailbox got too little data

Using 1.1.1. Got this error message in a tight loop, using both Tbird and Mulberry 3 as client:

FETCH for mailbox INBOX UID 32994 got too little data: 1616 vs 1649

I get this repeatedly alternating with a login line. What does this mean?

I stopped the server, zipped ~/mail/.imap/INBOX to hide it and let dovecot recreate it, and things seem to be working again. Is there a less destructive way to fix this in the future?

Delivery is by procmail.