I finally got the user and want to share some important steps with you
* There are some rabbit holes!
* Do a proper enumeration
* When you find out what kind of data can be "injected", you're probably on the right track, keep going, there…
It took me several hours to find the username. Now I finally got it. Hint for anyone else: You will see the username at some point. No need to search Wikipedia (like me), bruteforce or doing something else.