mailbox.log full of "invalid message sequence number" errors

[Lebha]

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

2009-09-03 17:24:30,897 INFO [ImapSSLServer-1443090] [name=first.last@domain.com;mid=2426;ip=1.2.3.4;] 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).

[Baylink]

I may be having a related problem: I'm getting that error from a perl deduper script... *on empty folders*.

Did you acquire a new user who creates nested folder where some folders only contain other folders?

I'm working with the author of delIMAPdupes to see which side it's on (which is to say that I've reported it to him, and patiently await a reply :-).