[Dovecot] Permission denied but works anyway

I've just updated from 99.14 to 1.0.a3 on a FreeBSD box. Everything went fine, except for a bit of flakyness from Thunderbird. When first checking a mail box in Thunderbird (running on Win2k), I get a message saying that the command can't be carried out and the server answered "Permission denied". I click OK and everything works fine. As long as Thunderbird stays open, the message doesn't pop up again. It only happens when I first check the Inbox for a given account.

No problems with Eudora on Win2k or mutt on FBSD.

I've googled with various keywords but I can't find anything on this. I have an extremely vague recollection of seeing a post on this, but I can't find anything.