The DW and Extension Manager are the newest versions I think.
The error message was just a translation. Here is the German wording:
"Die Menüs können nicht aktualisiert werden. Die Erweiterung wird nicht installiert."
I don't think it's a problem specific to my computer since I had the same error when first installing the first extension at all. There I could solve the problem by uninstalling other extensions (there were only two). But now I have the whole library installed, and that's why I ask again.