I'm really sorry to hear that the game did not work on Linux. Thank you for your thorough testing! I really appreciate it.
I was unable to test the Linux and Mac builds personally, and just compiled them in Unity. I still decided to release them because quite often they don't seem to need further tweaking. I hid the Linux build for now, so that other people will not have to be disappointed.
I can definitely remove the umlauts in filenames. Honestly, I should have done that to begin with. I can probably also find a way to mark the executable as Linux executable somehow. Unfortunately, the issue 4 seems like a much more annoying issue to debug, as the system works as intended in Windows. I will look into whether there are known differences in how the input fields work in Unity between different operating systems. If I find a likely culprit, I will upload a new version of the Linux build.
Again, thank you very much for your thorough report!