3.9.9 - Navbar unable to find the views when a main layout is used

I’m unable to set up the Navbar when I use a “main” layout for a simple site example:

Link to GitHub repository of my site: Broken site with a “main” layout

It loads okay initially:

It loads okay2104×1394 323 KB

But pressing the links breaks it:

Navbar does not work (tried to click Home)750×444 65.2 KB

Any help would be greatly appreciated.
–Ralf