If you receive this message attempting to join the server, just follow these steps:
1. Open your .minecraft folder (Please tell me you know how to do this).
2. Go into the bin folder.
3. Delete the jinput.jar file (Do not have Minecraft open at the time).
4. Start the Minecraft Launcher and click on options.
5. Click on Force Update.
6. Start up Minecraft, and play on our server!
If it still isn't working, leave the message it says when attempting to join the server.