Thanks Hans (and Till). Inspecting the log file did the trick (I didn't know it existed until now
) because it showed that mod_suphp was looking in the wrong place for the suphp executable. By default mod_suphp looks in /usr/sbin. However, on Debian suphp is installed in directory /usr/lib/suphp by default. Adding the extra option '--sbindir=/usr/lib/suphp' to the configure command of Hans' howto, solved that problem.
So it finally appears to work. I didn't do any intensive testing yet, but my phpinfo script and Roundcube site show regular web pages instead of an annoying 500 error.
There are still some minor issues to deal with though. An important one is the /etc/suphp/suphp.conf file. This file is part of the Debian suphp-common packge, but it differs from the one Hans uses in his howto and that's the one that appears to work fine with ISPConfig, so I would like to include it in the custom mod_suphp package. However, when I do that, I get an error message and installation of my package aborts. Unfortunately I didn't write down the exact message, but it had something to do with the fact that you can not just overwrite a configuration file that originally came with another package. I don't know how to solve that, so I hope someone can give me a few pointers how it should be done in a proper manner. Can you be of any assistance there as well Till?
Once I solved the last few issues, I'll post my custom package in the forum and maybe even write a small howto on how to use it. It will probably be sort of a short version of Hans' howto
At this moment I can't spare the time to set up my own package repository, but if anyone already has one and would like to offer this package from that repository, I would like that very much. Installing the package with aptitude from a repository is much more user friendly than using dpkg because it takes care of all the dependencies.