Try adding this to /etc/php5/apache2/php.ini
suhosin.session.encrypt = Off
This turns off suhosin session encryption globally and there are other ways to turn off suhosin session encryption only for roundcube if you'd rather put a .htaccess in the web directory of your roundcube installation.
If you are still having problems change the default session lifetime in roundcube's main configuration file from the usual "30" to "50".
Also make sure the following package is installed:
Hope this helps.
Last edited by CSsab; 27th August 2011 at 20:23.