ok, I figured something out:
The error differs with different PHP versions. In my webspace-config I can choose different php-versions. Using 4.1.2 the error described before appears, newer Versions of PHP show the following:
Failed opening required '/mnt/ja1/04/887/00000011/htdocs/geeklog/system/classes/timer.class.php' (include_path='.:/opt/RZphp4/includes'

in /mnt/web4/22/63/5887863/htdocs/geeklog/public_html/lib-common.php on line 158
???