ovoiS Haven't been able to be at my main machine (which is the one with this error) for a while, so I haven't been able to do any further troubleshooting.
The launcher works perfectly for me on a completely clean Windows 10 installation using Bootcamp on a Mac, although I was also on a different network to my main machine. To me, the error seems network-related but since even a VPN didn't help me, idk what else I would even try. Tethering from mobile, maybe? Of course, it might also not be network-related.
When I can next get to my main machine, I'm just going to try and copy my installation over from my Mac since I've got a working setup here. Maybe you could do the same for your friend – just supply them the files they're missing, if that's possible. Or maybe they could spin up a VM and see if it works in there, and then copy over the installation to their proper Win10 install.
It would be nice to have a way of installing Plutonium manually that bypasses the launcher, for users who know what they're doing or in case the launcher doesn't work (but I understand that it's much easier to do everything with the launcher)