This usually happens because of coding errors in the servers registry (not the servers fault but the plug-ins fault. In simple terms, it thinks that you are still logged in when you aren't. This is something that doesn't happen much but occasionally. Things I believe you can do to help with this error and others is by having an updated Java, and allowing MC permissions through your firewall or simply just restarting MC and re-logging into the server will usually fix the problem if not the first time the second or third. But do keep in mind like I said before, it isn't the servers fault, it's MineCraft itself.