@hmartens@gmail.com:
I was facing the same issue as yours. I resolved it by f...

@hmartens@gmail.com:
I was facing the same issue as yours. I resolved it by following:

1. Override the bootstrap page.html.twig as per #7 in your sub-theme.
2. Override the menu.html.twig file in your sub-theme in such a way that the Bootstrap's default dropdown functionality for the mobile menu is removed. (Refer Core Classy theme's menu.html.twig for reference.)
3. That's it, Clear your caches & Menu should render now.

Possible Issue:

• Bootstrap is adding 'dropdown' classes in menu.html.tag which is related to bootstrap's Hamburger menu.
• Bootstrap's dropdown JS is conflicting with mmenu's JS actions. Which is the reason we are not able to see the menu links from panel mm-2 onwards.

Hope this will help.