Hi,

You may also need to check that you don't have items with duplicate name in the menu. Remove the duplicates (even if they are in different menus).

Does that mean to not have 2 Menu-Items white the same name, or to you mean white duplicates just the section/category names?

If you don't have section listings (content_section) published in the main menu but use blogs instead (content_blog_section) items will inherit their Itemid and you will get Blog/Item concept pathway.

What does that mean? Because in my main-menu is nothing what is in my sidebar.