wordpress ‘database update required’ loop after update

I tried to update my WordPress version but it didn’t work.
If I log in I get the message

Database Update Required. WordPress has been updated! Before we send you on your way, we have to update your database to the newest version. The update process may take a while, so please be patient.

After clicking Update now I get the Message

Update complete. Your WordPress database has been successfully updated.

But after clicking on OK I get the first message again.

After reading a couple of threads I tried this:

• clearing browser cache and cookies
• checking PHP Version
• renaming/deactivating plugin-folder
• renaming/deactivating WordPress installation folder
• restoring WordPress and Database from my backups
• attempt a database repair with define('WP_ALLOW_REPAIR', true); Site stuck in "Database Update Required" loop
• I don’t have a db_upgradedfield in my /wp-include/options.php so i couldn’t set it to 0 Site stuck in "Database Update Required" loop
• I can’t find the db_version value under wp_options in my database. So I can’t check it like here WordPress 3.9 Database Update Infinite Loop

but nothing worked. I still can’t get to the backend.

Could anyone help me with this problem?