I actually am running Win10 with Microsoft Edge. Well since I have upgraded my OS from Win7 to Win10 I got both browsers:
Microsoft Edge and Internet Explorer. Both are up to date. So there's no reason for this.
Second one: I use WinRAR to unpack anything. It's also compatible with 7zip archives. Since other persons on this thread already used WinRAR and played the game successfully, it can't be the problem.
Since wolfzq also has no clue, what's causing the problem: "The final reason MAY be the following two reasons:", I'm gonna sit this out and wait for the next update.
But thank you much for your help so far.
Edit: Welp ... Something fixed the problem. I don't know, what caused the problem, but yeah. I think it was the new packed js-folder a few posts above mine, that fixed the problem. But I really don't know.