Another thing:
It´s equal if you setup your vCenters in Linked Mode or not.
If you try to create a category on one of your vCenters you cannot create it on a different vCenter with the same category name again. 
In my case, all vCenters have the same categories, so the import wizzard will fail on import, because the category "already exists".
A very bad situation for customers who have setup their environment like mine.
Thanks, Johannes!