Main Menu First Menuitem not working on Safari and Firefox

Hello There,
I have an issue with my mainmenu. in chrome everything works fine, safari and firefox linking always on startpage (if i click on the first menuitem, the other working correctly.
in detail: (only safari and firefox)
if you are on the landingpage (see private content) and want to click on “artmanufaktur” then it´s not linking to the “artmanufaktur” page.
all the other menuitems working fine.
If you need admin access, please let me know!
Thanks for support.