[Dovecot] Internal login failure

[Peter Clark]

    I have discovered an odd problem; some users are able to connect to 
Dovecot either via pop3 or imap without any problems. Other users, however, 
aren't, and the following message appears in /var/log/mail.log:
---
dovecot-auth: Login process 13 connected
dovecot-auth: Login process 13 sent handshake: PID 1313
pop3-login: Internal login failure: user at mydomain.tld [195.242.16.30]
dovecot: child 1311 (auth) killed with signal 11
---
I'm using Dovecot 0.99.11 (Debian Sarge), and all user information is stored 
in an LDAP database. Any suggestions?
    Thanks,
    :Peter