I use php for delivery cache files so there are no special rules. But of cause I deleted all custom rules in the htaccess file, deactivated all plugins execpt Wordpress SEO but the error still appears.
Switching to PHP 5.3 solves the problem. All custom rules activated again, all plugins (incl. WP Super Cache) activated -> all is fine.
That's why I am not convinced that it is not a problem of the plugin!
Now I just have to pray that I am allowed to change the php version on all other webserver where I am facing and certainly going to face the problem...