Problem is my installation was Windows 8: Single Language, not Core. I tried ei.cfg methods, not helpful. I found my key through a tool. But it does not even allow me to enter. With ei.cfg tricks I could enter but it did not accept. In Pro, it did not even ask for a key and said that I just can't install. Even supplying it with ei.cfg does not help.
Ah, I am not an expert on single language versions of Win8. It is not surprising that they won't let you install or activate any other version of Win8 using this key, but the installer just failing instead of prompting for a key looks like a bug. But as I said above the solution you found isn't too bad anyway.
