Using menu replacement plugin makes burgermenu to not appear in mobile

Hello Support
We are using a plugin (Dynamic Menu Manager by duogeek) to replace the main menu in the header depending on courses that the visitor has subscribed to. This is working without any problem for over a year but I just noticed that -while the replaced main menu does get hidden on smaller browsers- the alternative Burger-Menu does not appear in its place, leaving the website un-navigatable on mobile devices. This is horror!!!

Any Idea why is the BurgerMenu not showing when the (replaced) main menu gets hidden?

Thanks for your help.
Gregor