Category Link ends in 404 Error

Hello,

i have just noticed something strange on our blog (http://buechereule.de/blog/). We have hierarchical categories, but if you click the top-level category (like “Bücher”) we receive an 404 error. The categories without subcategories work fine (like “Leserunden”). Does somebody know what this is all about? We just can’t figure it out?

Thanks a lot in advance,
Jenny