Can you check whether you use zend encription or ioncube encription?
The 3.5.5 -> 3.5.6 update for ioncube: http://files.shopware.de/download.php?package=update_ioncube_3.5.6
You definitely need the ioncube version to update to SW4.1.2, but you should check what you are currently using.
The file needs to be unzipped to the shopware folder and the sql-file needs to be imported to your database.