Upgrade to 2.7 fails with Error 500

I’ve tried twice to upgrade from 2.6.3 to 2.7. both attempts failed, throwing a error 500 each time. Following the extended directions carefully did not help.

The instructions advise to delete the wp-admin folder. Later, they say to initiate the upgrade by pointing to: https://example.com/blog/wp-admin/upgrade.php. The problem is, that wp-admin folder was updated and the not recreated when the upgrade files are FTP’ed to the site.

Looking at the .zip file, there it is a flat structure (no folders at all) and does not include an upgrade.php file.

Am I doing someething wrong?