Can't add new category in support plugin

Dear support team

I need to run the support system plugin in my multisite installation. Everything seems working fine, except that I'm not able to set new ticket categories: if I give a name for a new cat. and press the button "Add New Category" the plugin confirms with "Category added" but I cannot see the new cat. in the list.
I've already deleted the plugin and the database, but also after a fresh installation I have the same problem. Any idea what I can do to fix that bug?

Thanks a lot
- Dominic