Ok checked and monitored maillog with tail -f. On attempting to login this gets logged:
Jan 7 01:24:38 svr1 roundcube: MDB2 Error: unknown error (-1): _execute: [Error message: Unable to execute statement] [Last executed query: DELETE FROM session WHERE sess_id=?] [Native code: 0]
Jan 7 01:24:38 svr1 roundcube: [07-Jan-2011 01:24:38 +1100]: DB Error: MDB2 Error: unknown error Query: _execute: [Error message: Unable to execute statement] [Last executed query: DELETE FROM session WHERE sess_id=?] [Native code: 0] in /var/www/roundcubemail-0.3.1/program/include/rcube_mdb2.php on line 627 (POST /webmail/?_task=&_action=login)
Jan 7 01:24:38 svr1 roundcube: MDB2 Error: unknown error (-1): _execute: [Error message: Unable to execute statement] [Last executed query: INSERT INTO users (created, last_login, username, mail_host, alias, language) VALUES (now(), now(), ?, ?, ?, ?)] [Native code: 0]
Jan 7 01:24:38 svr1 roundcube: [07-Jan-2011 01:24:38 +1100]: DB Error: MDB2 Error: unknown error Query: _execute: [Error message: Unable to execute statement] [Last executed query: INSERT INTO users (created, last_login, username, mail_host, alias, language) VALUES (now(), now(), ?, ?, ?, ?)] [Native code: 0] in /var/www/roundcubemail-0.3.1/program/include/rcube_mdb2.php on line 627 (POST /webmail/?_task=&_action=login)
Jan 7 01:24:38 svr1 roundcube: [07-Jan-2011 01:24:38 +1100]: PHP Error: Failed to create new user in /var/www/roundcubemail-0.3.1/program/include/rcube_user.php on line 451 (POST /webmail/?_task=&_action=login)
Jan 7 01:24:38 svr1 roundcube: [07-Jan-2011 01:24:38 +1100]: PHP Error: Failed to create a user record. Maybe aborted by a plugin? (POST /webmail/?_task=&_action=login)
Jan 7 01:24:38 svr1 roundcube: MDB2 Error: unknown error (-1): _execute: [Error message: Unable to execute statement] [Last executed query: DELETE FROM session WHERE sess_id=?] [Native code: 0]
Jan 7 01:24:38 svr1 roundcube: [07-Jan-2011 01:24:38 +1100]: DB Error: MDB2 Error: unknown error Query: _execute: [Error message: Unable to execute statement] [Last executed query: DELETE FROM session WHERE sess_id=?] [Native code: 0] in /var/www/roundcubemail-0.3.1/program/include/rcube_mdb2.php on line 627 (POST /webmail/?_task=&_action=login)
Jan 7 01:24:38 svr1 roundcube: MDB2 Error: unknown error (-1): _execute: [Error message: Unable to execute statement] [Last executed query: INSERT INTO session (sess_id, vars, ip, created, changed) VALUES (?, ?, ?, FROM_UNIXTIME(1294323878), FROM_UNIXTIME(1294323878))] [Native code: 0]
Jan 7 01:24:38 svr1 roundcube: [07-Jan-2011 01:24:38 +1100]: DB Error: MDB2 Error: unknown error Query: _execute: [Error message: Unable to execute statement] [Last executed query: INSERT INTO session (sess_id, vars, ip, created, changed) VALUES (?, ?, ?, FROM_UNIXTIME(1294323878), FROM_UNIXTIME(1294323878))] [Native code: 0] in /var/www/roundcubemail-0.3.1/program/include/rcube_mdb2.php on line 627 (POST /webmail/?_task=&_action=login)
Those question marks in the insert statement are a bit of a worry......