Recently our mailbox.log has been filling up with entries like this:

2009-09-03 17:24:30,897 INFO [ImapSSLServer-1443090] [;mid=2426;ip=;] imap - S: 2660179.624 BAD parse error: invalid message sequence number: 1:*

Before the problems, mailbox.log used to be ca. 50MB per day. Since the problems appeared a week ago, log file size jumped up to gigabytes per day. This errors seems to appear on about 20 of our 800 users.

What could cause this weird behavior? The affected users don't seem to notice any problems with their accounts (with one exception: one user whose messages are multiplied when sending to certain other user).