By the way, I already cleared the CSS cache in Mega Menu > Tools and also I already tried to apply the solution mentioned here:
https://wordpress.org/support/topic/menumenu-displaying-in-header/
Nothing helps.
I just found the solution myself:
I went to Mega Menu > Menu Locations > Mobile (optional) and I clicked on the little arrow, then I got this message:
“Max Mega Menu is not enabled for this menu location. If you wish to use Max Mega Menu for this location you must enable it under Appearance > Menus.”
So I did that and both problems are solved.
Thank you for such a great plugin.