Usually, one of two things causes an "Unexpected Error" message.
1) Your connection speed is too slow, or something is interfering with your connection. Be sure to check that no one is using or hacking your connection, streaming videos, or downloading torrents, which can cause the game to malfunction.
2) An error is occurring on the developer end. In this case, everyone will be experiencing the issue and the developers will be working on it as soon as possible.