If you experince immediate logouts after successfully loging into roundcube, you have to follow the advice in /usr/share/doc/roundcube-code/NEWS.debian.gz and turn suhosin.session.enrypt off [1]. If you do not want to turn it off globally, it can easily be done for roundcube only:
/etc/php5/cgi/conf.d# cat roundcube.ini
; see /usr/share/doc/roundcube-code/NEWS.debian.gz
[suhosin]
suhosin.session.encrypt = off
[1] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=549428