I spent the last three days trying things and searching via Google, but I’m not a single step further to a solution.
Is there nobody who can help me?
EDIT:
As always, as soon as I ask about help, I get new ideas.
Even if the source code of another extension (I’m currently using, but because it doesn’t work as I need it, I decided to write an own one) is not public, I just came to the idea, that the code might not be minified. And yes, it isn’t.
So I searched through the source code of the other extension and it does make requests exactly like I do. So this can’t be the problem.
That led me to another question, which might solve my problem: Is it possible, that the Twitch mobile app disallows requests of mobile extensions, which are not released, yet?
I guess I’m trying that by just release the extension and see if it works after.