I forgot to include the Links how I found out about the solution to disable the gzip option:
what really helped was:
http://stackoverflow.com/questions/622192/php-warning-headers-already-sent-in-unknown
especially the second answer. I don't know if that, and the linked bugreport there:
http://bugs.php.net/bug.php?id=9031 are really the cause (the Bug is really really old), but maybe it is a hint :)