Basics not working - what is the problem?

Hello

I worked through Ben Pleysiers super Node JS series.

However, the Register-Form is not writing the entries to the db. I started debugging with help of DevTools as recommended here: Debugging Server Connect Errors

This brought me to a much more basic problem: In DevTools / Network / XHR Tab no activity is recorded. But even worse and more basic not even basic links of the main page to static subpages are working. In DevTools / Console these messages pop up when I load the page:

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So I believe something very basic is not working here. Anyone an idea what I have to do in order to get the system in its basic up and running?

System check, green dot etc. is all OK.

Thanks for any hints -

Thomas