Dear David and Cactus,
I have to say, the problem seems to be solved by upgrading PHP5-CGI module to 5.2.8-1 from the contrib on this site:
http://wiki.contribs.org/PHP5Even the serverload has gone down with 20% !
See screendump:

No more log errors even after changing the log level back to "warn" (as it is the default setting).
So I really thing that the manual on this site has to change!
http://sme.firewall-services.com/spip.php?article46 I think nobody should use this contrib anymore, the new contrib is even easier to configure and has a nice template just as PHP4 has!
Thanks again for your support and the good idea's !
Have a nice Easter holiday!
grtz Simon