more info:
My mail log /var/log/mail.info shows that when I attmept to login via RoundCube, it considers the login successful:
Oct 24 22:47:01 myserver imapd: LOGIN, user=user@domain_name, ip=[::ffff:192.168.1.2], protocol=IMAP
Oct 24 22:47:01 myserver imapd: LOGOUT, user=user@domain_name, ip=[::ffff:192.168.1.2], headers=0, body=0, rcvd=25, sent=180, time=0
Unfortunately, RoundCube doesn't agree!