Thank you for your answer. I changed the URL to https://unpkg.com/vtk.js@8.4.2/dist/vtk.js now, to make sure it is the current version. This works in a small test-HTML without any issues but in my real application, I always get some older version it appears (I’m also missing some other functionality).
Since I’m working with typescript, I import vtk via a script tag in my HTML and access it by declaring
declare var vtk: any;
This lets me access the import in my code, but as I said, apparently an “incomplete” or old version 
I will keep trying to figure out why. I would have wished it was cache related but it does not seem to be