Thank you very much for your reply. I’ve seen it only today. Unfortunately, I have not fixed my problem at the moment, but I believe I will fix it soon.
My additional problem is that my php.ini overwrites the default one, i.e. when I create a custom php.ini file, it disables a lot of settings of the “master” php.ini file, even though I am not overwriting any file. When I remove my custom file, the original settings are restored. Therefore, I could backup the master php.ini file (using this guide: https://drupal.org/node/290592 ) but if I use that file I receive a 500 server internal error :S
I am currently trying to figure out which line is causing the error, I hope I will get by it soon.