Thank You very much.
That was the lead that I needed. The problem was between the chair and the keyboard.
I removed the PHP filters from /root/ispconfig/scripts/lib/config.lib.php, because the combination of PHP & Apache2 in Suse10 caused a constant error when accessing PHP pages. The error did not cause any harm, but it did badly polute the logs with one error entry for each access. In the process I introduced a syntax error in config.lib.php.
I will test the integrity of the system for a while, but so far everything seems working well.