Struggling with Menus

Recently I’m struggling with adding / saving Menus while creating a new GP site.
I create a menu, add some pages, select the locations (primary / off-canvas) but then, the Menu doesn’t appear and when going back to the menu the check-boxes of the locations are no longer checked… it’s not saved.
In the header appears another “Menu” (not my menu) with all the pages for this language (autom. adding new pages is not checked).

– This happens both in the general WP Menus option and in the GP customizer Menus option.
– It happens both with the GP Pro Module Menu Plus on and off.
– I use Polylang for the languages.
– I disabled Cache.

Are you familiar with this problem?