Well it seems like you're only having trouble with that particular server, it could have been a server update causing you to be outdated whether it be a new snapshot or just a force update on the server. Try resetting your connection.
I have, a different issue. My minecraft keeps saying that its unable to connect to a server due to um...im gonna have to check again or something...I have resolved the problem but I do want to know an explanation to this matter.
And as a reply to your situation, I think the servers can be overloaded sometimes...because I play on a server called " TeamExtreme" and apparently its always full and you may have a slight chance of playing for awhile. I have already tried asking them to increase the maximum amount of players on the servers but it seems there are other more serious matters at hand for them.