I was playing with friends using e4mc, but when I loaded chunks with the forlorn hollows in them, everyone immediately got kicked with the usual Disconnected error (no relevant logs appeared in anyone's console). If they tried to rejoin while forlorn hollows chunks were loaded, they would be able to see the world for a few seconds before being kicked again and I would receive an error occurred in e4mc.

On my host machine, I did notice that the TPS was much lower than usual when loading forlorn hollows chunks, though people were connecting at the same time and I had a lot of mods like Simple Voice Chat that would've been doing handshakes at the same time, so it's not clear whether the forlorn hollows was causing the lag. There wasn't any FPS drop, just TPS.

Switching to Radmin for LAN connection, the same issue persists. I don't have a server to test it on.

Alex's Caves: 2.0.2
Loader: NeoForge 47.1.106 (Minecraft 1.20.1)

A small additional note: This seems to be only related to the forlorn hollows, as we accidentally passed over a candy cavity (saw a random gum worm on the surface) without any issue on our way to the forlorn hollows.

Btw the disconnect is not instantaneous, my friends reportedly started getting bungeed for a few seconds before the disconnect message popped up. It did seem too fast to be a connection timeout though.