Hannah Banana

I was wondering If someone could help me with this modpack. I have a minor problem. The modpack it self seems to run/play fine. I have had a world for a long time that I have been playing with my brother. But for some reason, now when I start the LAN he isnt able to join my world, and I crash. When he trys to join he gets the message "internal server error" and im taken to the Minecraft has crashed screen. The game works fine, and I have played by myself for hours, its only when he joins stuff has crashed. I have not tried this with anyone else. What do I do???