Answered No installation possible: mod_fcgid: stderr: PHP Fatal error:

Ok. after 3 Days of investigation, i got it.

Disabling Optcache in the Options of the website does it.

Custom PHP.ini settings: opcache.enable=0

I think the problem is the flushing of the cache with too much data in the install process. Because of that i got various error messages and the error

"End of script output before headers: index.php"

gave me the hint i needed.

Now i can go to the login page

in ispconfig /website/options put this in custom php.ini settings: opcache.enable=0

that disables the cache module and the installation goes flawlessly, shiny and pretty