it seems it has something to do with the easyphp version. Initially I installed 1.9.7, later upgraded to easyphp 1.8 but it appears that i still have a version 1.9.7 of categorize_list.php.
Notice: Undefined variable: categories in c:\programme\easyphp1-8\www\tikiwiki-1.9.8\categorize_list.php on line 35
Any help is very much appreciated!
BTW: I have installed TikiWiki locally on my machine on Windows XP Service Pack II