sorry for posting my question into a resolved topic. 
That's why I made a new post:
I am using wp_list_categories() but the categories aren't listed.
They are in the database, though and show up when creating a post.
They don't show up on the categories manager.
Is there any way to debug this?
It worked fine until a server reboot.
an additional note:
adding a new category on the edit post page removes all listed categories from the list ... the new category is displayed until it is selected.
After saving the post, the category vanishes.
Same for the category manager.
The new categories show up in the database.
I am using wp 2.7.1
Any tips are apreciated.