i have taken the base FTB Horizons modpack from the curse launcher, and tried to create a custom pack based on it, by just adding some mods. after a lot of messing around i managed to set up a server and a client which both have the same mods and configs, except for client side only mods, which are only on the client.
i am able to run a server, and a client, but when i connect to the server one of two things happen:
1. i immediately get disconnected saying: "a fatal error has occurred this connection is terminated".
when this happens, i can't connect no matter what, and only fix is deleting the world.
2. i can connect, and i can walk around, but after a little bit of exploration, i get kicked with the error: "a fatal error has occurred this connection is terminated", and can't reconnect anymore, leading me to believe that this is an issue of a corrupted part of the world being loaded.