Thanks for your reply, JabbaTek.
I have repeatedly tried to login since installing RoundCube (- Powweb's automated install script where I just had to fill out a few fields and click install) using any of the active email accounts which are all working with SquirrelMail. The database is MySQL v.5.0.45, and the installed Perl is v.5.8.8 and PHP is v.4.4.7. All running on a Debian Linux server.
It almost appears to me that the accounts are not being created in the MySQL database even though the auto create user code in main.inc.php is set to:
$rcmail_config['auto_create_user'] = TRUE;
Any pointers to resolve this would be greatly appreciated.