I saw this error in the log; the server is unable to verify your account with Mojang. It could of been an account server issue (outside our control) or an issue with Essentials… I blame Essentials. I have checked and verified your account does return as valid from Mojang (an UUID is assigned) so it appears the issue is on our side.
Are you able to log into other servers that are operating in online mode (only allow valid Minecraft accounts)?
I have updated the user file with your actual UUID (which is how servers are supposed to track your account). You could try logging in again… otherwise I have to hit things with a hammer.