Thank you all for your replies.
I finally found the solution.
The problem was not caused by WordPress or the webserver. The problem was that after updating my FTP client, a default setting was changed.
The setting in question was “Convert file names to lowercase”. As a result the server was not able to find all of the files required to launch WordPress…
It was not easy to find this solution. It would be nice if WordPress did not have filenames with capital letters – or if the setup-config.php would do a check to see if files have the correct upper or lower case letters.
