[Dovecot] assertion failed: (fd >= 0)

After I activated disabling POP3 for one of my users (not user 'jos'), the following is in my logfile:

Dec 3 16:03:33 webrz dovecot: IMAP(jos): Disconnected Dec 3 16:03:36 webrz dovecot: IMAP(jos): Disconnected in IDLE Dec 3 16:03:36 webrz dovecot: IMAP(jos): file ioloop.c: line 22 (io_add): assertion failed: (fd >= 0) Dec 3 16:03:36 webrz dovecot: child 74197 (imap) killed with signal 6

Can someone tell me what is going wrong? Have a deny.imap file with no users in it and a deny.pop3 file with one user in it...

Jos