Thank you very much for your support.
Currently I only have one menu (translated with WPML into two more languages) and marked as main menu and slideout menu.(Aside from the language selector).
I tried to create a new menu just for the slideout menu but the problem is the same. I have only detected that in that case the menu repeated below is not the slideout menu but the main menu. I don’t know if this can help to solve the problem.
I have looked at the documentation you send me and it seems all right.
I can’t be sure when the problem occurred but it could be related to the GP Premium 1.7.2 update I made this week. It used to work correctly and the menu settings have not been changed.