SPFx extension examples are not displayed

I have tried several examples of SPFx extensions. All samples have the same result with respect to the tests (swallow serve) after following the instructions on the examples page.

Most of them just need npm install and adjustments in the serve.json file. However, every time I test it, the extension is not loaded. I will not have any error or anything, but it does not load / do not run.

Even though I am the helloworld tutorial of the series of official tutorials, the extension does not show up (with the exception of the very first step alert).

I have also tried on different machines and on different networks, but I have never managed to get samples to do anything.

does anyone have an idea?

Version Node.js: 10.15.3
npm version: 6.4.1
gulp version: 2.1.0

Edit: If you try with NodeJS 8.16 LTS, the same problem arises.