Attempted to connect to a server as JoshBlake that I had previously connected to from another machine as JoshBlake. I'm guessing the user token was different, but this is the message I received directly after the "Handshaking..." message:

Connection error: EndRead failure, An established connection was aborted by the software in your host machine.

After changing my username to something else, it allowed me to connect. (I had already verified I had the latest/same client version.)

I think the error message if your username token is wrong should be more user friendly.

This sounds related to #83...

Confirmed fix at of e386bfb. Message now says "Handshake failure: Invalid player token for user"

Eventually a FAQ about how to fix that message might be helpful for the broader user base.