There seems to be a bug in the virtuser_file plugin:
[23-Apr-2010 15:33:07] PHP Warning: file(): Filename cannot be empty in /var/www/webXXX/html/roundcubemail-0.4-beta/plugins/virtuser_file/virtuser_file.php on line 90
Looking at line 90 of virtuser_file.php and comparing it with the respective part of the stable rc-3.1 release (rcube_user.php line 536) I changed the following to make it work again:
// if ($this->file)
// $virtual = file($virtuser_file);
if ($virtuser_file = rcmail::get_instance()->config->get('virtuser_file'))
$virtual = file($virtuser_file);
I am sure a real roundcube dev is able to fix this properly :-)
Fixed Changeset 3560 ? Roundcube Webmail (http://trac.roundcube.net/changeset/3560)