404 error for me as well with UE connected to github
Edit: For those who have problem after me, who also have little or no github experience like me -
The “Epic Games” Team from Github will send you an invite to join the Team, will allow you access.
I believe the process is automatic, but the invite did not appear prominently for me until I followed a completely unrelated link to another UE Git repository, got the same error, then googled the team and went to Epic Games · GitHub where the invitation was in a bar at the top of the screen.
now works.