Thanks for replying. Good to know i'm not the only one. :)
You may be right with your webserver configuration assumption.
Somebody gave me ahint that it could be a MIME type thing.
See: https://forum.unity.com/threads/webgl-netcoreapp-1-1-unityloader-js-syntaxerror-...
But if that's the case there is nothing we can do about it on our end, right?