Can't enter my minecraft server

Hi, my friend has recently had problems entering our server. Each time he tried he got kicked out yet I can still enter. We have a modded server and we recently got a new mod. The Rc Mod. Clearly it had to be something to do with the mod. We checked that he had the correct version, deleted the mod and installed it again yet he still can't enter. This only happened after mod was installed. I entered the server an asked him to try and join. He entered but left. I told him that he left ,but he says he wasn't even in yet, it was still loading, shortly after this message popped up on his Screen: "Internal Exception: Java.io.IOExecption: An existing connection was forcibly closed by the remote host". I copied this directly from his screen. I'm not exsactly sure what it meant but I told him to use task manager and close end any task with java or Minecraft on it. He did so and we tried again and we got the same result. All I know for sure is, this happened only after the "The Rc Mod" was installed.