Cannot connect to anything but the Rebel City (SBA)

Q: Whenever I launch the game, I start out in the Rebel City just fine, but when I try to leave the Rebel City one of these things happens:

• If I fly up to one of the heli-carriers (which should transport me to the overworld), it says "Reconnecting" and then the game eventually gives me an error and closes.
• It says "Joining Server" for a few seconds, then says "Finding Rebel City", and then I'm back in the Rebel City.

A: The player who reported the problem later said that their anti-virus software was causing the problem.