I have the same problem, but not on a server. Trying to get on a LAN, same version I downloaded both versions at the same time, same version of Minecraft. When I go to join the LAN game, the server side gets a notice that I disconnected, (never that I connected) and client side gets either "Connection Lost" or "Fatally missing registry entries". I've tried doing a LAN with other modpacks and it works fine. Re-downloaded everything, still getting the issues. When I clear it through Twitch are there other configs I have to manually delete? Everything I could find related to the pack was gone.
