First thing to do when you run into anything like this is to press F12 to open your browser's developer tools and look in the console to see if you have any errors.
In this case you'll see that when you change pages you've got the following error:
TypeError: this._blockModels is null
Unfortunately it's 'black boxed' so the source code can't be viewed. If you build your course with $ grunt dev then you will be able to click on the code reference next to the error to jump to where in the code the error is.
However, I do know that
_blockModels is a property in background-switcher so it's most likely to be something wrong with that. Best to check you definitely have background switcher configured properly.