I tried this link:
Willkommen->Über Uns
This took me to a page where the menus worked.
Then I tried this link:
Willkommen->Kontakt
The menus didn't work here because the document displayed on the "Frame25956" frame did not contain the "frames loader code".
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Remember that on a frames implementation all the documents that are to be displayed on the same frame as the menus MUST contain the frames loader code.
When you enable the frames support in DMB the menus are split in two parts. One of the parts contains the toolbar and the other part contains the menus.
So, the only way for the menus to work is ensure that both parts are loaded.
The toolbar is loaded by installing the loader code and the menus are loaded through the frames loader code.
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I also noticed that most of your links point to pages outside your domain. Because of a limitation implemented into all the browsers, the menus will not work if the document loaded into the menus frame is located on another domain.
For more information
here's a very useful article that explains why javascript code cannot access frames containing documents from different domains.