I am also a customer of all-inkl. I am owner of a server there and run about 10 WordPress-blogs connected with different domains. Interesting: 8 of 10 updates work fine, but 2 does not. Same problem updating themes and plugins as described several times.
All installations are without any special htaccess-code about php-version, so all are PHP 5.5.22.
I have compared htaccess-files on working and non-working installations and can not see any differences.
Hmmm. -?-