Bad callsign - already taken (Multiplayer cannot start)

[SantasGolfClub]

I am trying to learn with an online mate, and I started a server on my local machine. I opened the port (Not sure if I have to, but I  did), and sent him the IP. Because it was public, he actually saw the server on the server browser.

But as soon as he tried to join, he got the message: Bad callsign - already taken

Someone else (A random) did manage to join. So I believe it's the other chap trying to connect to me that has the issue. Has anyone seen this and know what the fix is?

[BIGNEWY]

check he has set a MP name in his settings. He may have tried joining as the default name 

[SantasGolfClub]

Thanks for that! I noticed mine was on default too, to 'Player'. I've messaged the chap, and hope this is the solution. Thanks! You seem to be involved with the game somehow. Maybe an 'Username in use, please use another one' type error might be easier? 

Thanks for the help.