As reported at http://www.cafu.de/forum/viewtopic.php?p=5425#p5425, loading the Main Menu into the GUI Editor, moving a window, and saving again breaks the Main Menu.
It looks as if the GUI Editor attempts to prevent duplicated window names by renaming them, inherently causing the error.
We have to check if the name checking really should be done at a global level, as is now, or if checking the siblings of the current window is enough.
Alternatively, the problem might be fixed by updating
MainMenu_main.cgui the reflect the disambiguated names.