Thanks Deacon Frost,
I tried a fresh start and edited postings.php and index.php as described. Before that I dropped the modlog table and created it from scratch by copying the sql code into phpmyadmin (worked).
Copied modlog into /forum/admin directory and tried to have a look - works fine for the Administrator,
but not for Moderators.

Everything seems fine, except that the "Moderator" row is blank.
When viewing the db I see that the userid = 0 there
I don't understand anymore: I tried it now a bunch of times, same error everytime.
Any hints on what can keep me getting always the same effect.
Code seems fine...
thx,
-Tom