I'll mark that issue as resolved. Problem is, that the old Discussion Categories Block was modified for the new template whereas others, like Event Categories, Blog Categories or Space Categories are still using the old display method. Deleting the old Discussion Block and inserting it again, did the trick. I think, the dev team will modify all of the Category Blocks for the new visual display, too, with one of the next updates. See screenshot for explanation of the differences in showing the Blocks.