I have a very similar or even the same problem. When I tried with UE 4.20, the message is different, but results in the same, more or less.
When I tried generating project files with 4.19, I got the exact same message though, down to UnrealVersionSelecter.log file and the UnauthorizedAccessException to the cut part of the user name.
I just opened a question here, too: Can't Generate Project Files - Unauthorized access to other PC's accounts - C++ - Unreal Engine Forums
But it seems my guess was wrong. I had, coincidentally, a 2nd user with the very same name (in this case: ron), so I thought the system tried accessing the 2nd user. But if that’s not the case for you then that really just leaves the white space issue.
Can Staff clarify? This is a really big issue with no easy fix on the OS side. I think the UBT needs to be fixed ASAP.