I just figured out that Google Webmaster Tools shows lots of error messages. They come from wrong category names e.g. "example.com/android" instead of "example.com/category/android" (see here: http://mobiwatch.de/category-sitemap.xml)
In addition, there's a problem when I'm trying to add a category to my menu navigation (via "Design - Menues - Categories - Add to Menue), these links also say "example.com/android" (without the "category" in it's path). But those pages or urls aren't existing ...
I left the WordPress setting by default, just switched in "permalinks" the post links to "example.com/postname", that's all. The field "Category Basis" is left blank.
I really hope somebody can help me with that because I don't have a clue wthat to do!
Thanks in advance!!!