cannot connect, game already started

i'm sure this must have been posted before probably but sorry i couldn't find the post.

when i try to host a game and my friend joins the server stops and when i try to do the same i boots me from the server and gives me a message like "cannot connect, game already started" i found a solution to this before on a different computer (now busted) and i have to to delete 3 files because mine is a downloaded

anyone know what these files are called or know another solution to my problem?

oh and btw i'm running version 1.17 and 1.03