This happens sometimes when the server restarts or has a small burp, though it should be back up in a few minutes or less.
What I do is wait a few minutes before trying again if i'm in a public map with lots of people on it. Though, if I'm in a private map with very few inside, I would try again right away as it just will sometimes do that when it pleases for me. If it continues, then I would wait again for another few minutes. After that, it should be fine. C:
Happy FeralHearting!
~Wafflez