So it was really a very stupid mistake.

Our filestructure:

src/js/event-webcomponents.js

and in the package.js the module attribute was still pointing to the old name of the file: "module": "src/js/events-webcomponents.js",

classic typo...

Unfortunatley the typescript "Module not found: Error" message is not very helpfull in this case. But if you have a similar issue, it is worth to tripple check all your paths.