Clic on Start Menu.
Go to the folder where EB is located.
You'll notice that there are two executable links, Play Multi Player and Play Single Player.
Clic on Play Multi Player.
When you are inside the game, do a normal custom battle.

You should NOT instead clic "multiplayer" inside the main menu of EB, it's a different thing.

Many people get confused because, when they're facing the custom battle rooster bug and users suggest them to run the multi player version of EB, they think that they should start a multi player game inside EB. Nope, they should run the "Play Multi Player" shortcut instead of "Play Single Player" when they want to launch EB, BEFORE entering the main menu.