This is my plugin section (it's default):
$config['plugins'] = array('managesieve', 'password');
This is part of the log after trying to create any filter:
May 18 08:08:38 post roundcube: ERROR: connection refused (111)
May 18 08:08:38 post roundcube: PHP Warning: syslog() has been disabled for security reasons in /opt/www/roundcubemail-1.1.5/program/lib/Roundcube/rcube.php on line 1303
May 18 08:08:38 post roundcube: ERROR: Not currently in AUTHORISATION state (1)
May 18 08:08:38 post roundcube: ERROR: Not currently connected (1)
It is clear that there is not enough connection rights, but it is not clear which