News WebPart not working

When I try to add a new News Article on a communication site, the upcoming page is blocked. You cannot even click cancel or the upper right cross to close the form. Nothing happens.

The console shows a lot of errors:

You can also not choose one of the three templates.

What is also remarkable is that the start page of this fresh communication site (not published) is reporting errors in the console:

We are completely stuck here. Please have a look into it.