Troubleshooting upgrade from 2.1.3 to 2.3

I upgraded directly from 2.1.3 to 2.3. Followed the “four steps”. All plugins have been upgraded. The problems won’t disappear even if I deactivate all plugins.

Now I have two mysterious problems:
a) I can’t create a new category (via Manage > Categories)
b) I can’t delete a static page (via Manage > Pages)
The site itself works perfect for my visitors. I can also publish posts.

I am not very familiar with php, but have to solve this by myself. I know there is a problem with my wp-config.php file, but when trying to resolve this things get worse.

I am quite sure this has something to do with how categories are called. But that’s all I know.

Please explain in the simplest possible way if you have any advice.