It’s not a fix for the actual issue, it’s just forcing the handshake validation to skip verifying the client. They have this console command net.VerifyNetClientID but it doesn’t work because they made a mistake in the code:
So I’ll create a pull request for this which will allow you to use the console variable instead.
Edit: Someone already reported this and it has been merged in https://github.com/EpicGames/UnrealEngine/commit/1bd988a781c6762f641a09cedb81aafe4a1e428e