It's definitely interesting. The crash is likely being caused by interprocess-network communication, so it's likely a library that is crashing the game. There is no API (just checked) to get the full data path for the game with the username, so it's certainly not entirely the game's fault in this case for non-ASCII characters.
It's happening in the game binary though as I said previously, so it is definitely something that Mojang should be able to patch.
I renamed my username and folder following Method 2 here:
And Minecraft worked fine on this account too. So it's clearly related to the Norwegian character in my name.