The Ajax feature is causing the site to behave strangely.. It conflicts with RS Forms and eg DJ Image slider.
And internal linking to the contact form is creating errors in the form. But thats not all.
If "Show submenu-items" is turned off in the main menu module. The menu behaves incorrect.
For now I turned off AJAX on the complete site. This in combination of turning off "Show submenu-items" in the main menu module is the best option for me now. There are no conflicts with external software and the menu is working fine.