Menu on subsite doesn't work (upfront/fixer)

I'm using upfront/fixer on my client site - and besides tons of upfront/fixer-bugs I'm dealing with all the time, I have a big issue I can't handle. In general it is a one-pager site, I have just one subsite (for the imprint, see Link "Impressum" in the footer), and there the menu doesn't work probably. Whenever I try to link directly to the anchors of the homepage, the link doesn't work at all. Do I link only to the main URL itself, the anchors on the main hompage don't work anymore. So my question: How can I set different - working - menus for the homepage and a subpage?