Look at this thread, it also contains a message from K-9 maintainer - but unfortunately no solution:
Do you use IMAP? Then maybe clearing filders locally might help assuming that you still have all relevant mails stored on mail server
Look at this thread, it also contains a message from K-9 maintainer - but unfortunately no solution:
Do you use IMAP? Then maybe clearing filders locally might help assuming that you still have all relevant mails stored on mail server