Users unable to login

I have certain power users who can't login through POP3 or IMAP from their computer while being receiving mail on their cell phones (pop3), despite pop3_lock_session = no

Am I hitting some other limit? I don't see any error in dovecot.log but a generic failed login:

Oct 21 10:31:44 imap-login: Info: Disconnected (auth failed, 1 attempts in 2 secs): user=someuser@somedomain.com, method=DIGEST-MD5, rip=::1, lip=::1, secured, session=<XSVQLSqy2IYAAAAAAAAAAAAAAAAAAAAB>

Running version is 2.3.11.3