Can’t save IN categories (WP 2.6)

Hello,

I upgraded from WordPress 2.3 to 2.6.
After the upgrade I wasn’t able to see any category but I got this solved (http://blog.cumps.be/wordpress-26-upgrade-fix-missing-categories/).

The problem I have now is this:
When I publish/edit a post I select the ‘category checkboxes’ in which I would like to save my post.
But when I click on ‘save/publish’ the post is not saved in any of the categories I selected.

Any suggestion on how to solve this?

Thank you!