[Pkg-roundcube-maintainers] Bug#639950: unable to login

[Moonwalker]

Package: roundcube
Version: 0.5.3+dfsg-1

I'm unable to login to any mailboxes which were not used with Roundcube before.
There is a message in /var/log/roundcube/errors:
PHP Error: Failed to create new user in /usr/share/roundcube/program/include/rcube_user.php on line 473 (POST /?_task=login&_action=login)
PHP Error: Failed to create a user record. Maybe aborted by a plugin? in /usr/share/roundcube/program/include/rcmail.php on line 747 (POST /?_task=login&_action=login)

Expression "$user_id = $dbh->insert_id('users')" in line 421 of /usr/share/roundcube/program/include/rcube_user.php returns NULL in such cases, but it doesn't seem that the insert query actually fails.
Unfortunately, I'm not familiar with Roundcube code and PEAR's MDB2 to diagnose more deeply.