PHP bugged

[Mastacheata]

That sounds strange.
Just to clarify some things in my mind and get clear that we talk about the same thing.
Your error message still states an open_basedir restriction. The only difference is that it now states the correct path to your domain (e.g. /var/www/virtual/hosting-zone.eu/...)

That would be pretty strange since the open_basedir directive states just this path.

There's something even more strange in your last post. Your Apache config says something about mod_php5. Are you using a custom php install for that site? Maybe you are editing the wrong php.ini then.

Another Diagnostics Idea:
Inside the folder where the script resides that calls the file_exists function put a phpinfo document.
Compare the path that your script tries to open with the open_basedir path that phpinfo says.