[Dovecot] Errors in /var/log/maillog and /var/log/dovecot.log

I am trying to migrate from UW imap to dovecot.

OS: RHEL 3 X86 MTA: Postfix Auth: PAM (NIS server)

Most of the users are fine, but I have some users have problems:

" Jan 14 09:18:42 Info: imap-login: Internal login failure: user=<user1>, method=PLAIN, rip=X.X.X.X, lip=X.X.X.X, TLS "

This happens on dovecot version dovecot-1.0.alpha5-1 and dovecot-0.99.11-1

If anyone else has met such problem?

Thanks

Li