I set up a LAN world for me and one other person. When I, the server host, use the Floo Network, it immediately force disconnects the other player. When the other player uses the Floo Network, it works with no problems. I've tried setting up a dedicated server but that disables too many mods in a world I've gotten too far in. There is no crash report it simply disconnects the other player on the lan world.

Does anything appear in your / the others log when this happens?

I've tried setting up a dedicated server but that disables too many mods in a world I've gotten too far in.

Any mod that works in singleplayer should also work fine on a dedicated server. Client Side only mods like minimaps and stuff obviously don't but they aren't required serverside.