Hi Till. Thanks once again for your reply.
Because Hans' howto first disables mod_php but in the end of the article enables it again to support suPHP incompatible applications (which is what I need too), I thought it wouldn't be neccessary to disable it in the first place, so I didn't. Also I expected the Debian package to enable mod_suphp automagically when installing the package, so I didn't do that either.
I did a fresh installation and this time I first disabled mod_php. After installing the package, I explicitly tried to enable mod_suphp, but got the message: 'This module is already enabled!'. So once again all looks well. Unfortunately it still doesn't work. A simple phpinfo script results in the 500 error (the file permissions of the script are 644).
I can't find any related error messages in the apache the log files. I also still don't have a suphp log file. When you say 'the error log of the website', which log file do you mean exactly?