Ok.
So the main menu is called "mainmenu" per default. If i assign the mainmenu to "menu" it doesn't show up (It doesn't show up at all!) It doesn't help to delete the mainmenu and create it again.
If i assign the mainmenu to "footer_nav" it shows up at "menu and "footer_nav".
I now have Footer Menu assigned to "footer_nav" and Main Menu assigned to "footer_nav". Which at least displays the main menu links at "menu" position, if i move the menu items around. Still at "footer_nav" there is the footer menu items with a few "mainmenu" items.
Teitbite, if you like you can get admin access to have a look. The thing is that this problem needs to be resolved or i cannot use this template