Sorry to ask again some stupid questions but I haven't find any log files to fis this error:
I added a new domain, restored everything from the backup (like I did 4 times before) and the backup restored everything fine. After checking if everything works I got a problem. One subdomain sends the php file instead of executing it on the server.
I removed and added the subdomain - same effect, removed and added the whole domain - same effect, did the same without using the backup - same effect.
It's just this one subdomain. All other domins/subdomains work fine (yes php support is on).
Log file at /var/log/apache2/sub.domain.tld/error.log is empty and there is no log file at /var/www/virtual/domain.tld/logs/