Okay, I updated PHP and now Zend OPcache is fine.
But I still get the '500 Internal Server Error' message. I tried also to change the file permission of index.php, but this didn't solve the problem.
My error log:
Request exceeded the limit of 10 internal redirects due to probable configuration error. Use
'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a
backtrace., referer: https://forum.xxxxxx.de/