Did you change the perms?
As root execute these commands:
chown -R hestiamail:www-data /etc/roundcube/
find /etc/roundcube/ -type f -iname "*php" -exec chmod 640 {} \;
chown -R hestiamail:www-data /var/lib/roundcube/
chown -R hestiamail:www-data /var/log/roundcube/