I did a fair amount of troubleshooting before I got here, such as testing different versions of Forge, removing mods, etc. but I have run out of motivation. Sorry if a fix for this is listed somewhere and I have failed to look it up.

The title pretty much describes this issue. A log of the crash is at https://paste.ee/p/veCYu I'm happy to do any additional testing I may have been unaware of.

I did the following steps to reproduce it:

• Create a new empty MultiMC instance, choosing Minecraft version 1.16.3 for it
• Install Forge 34.1.0 (the 'recommended' version, but some other versions including the most recent one also crashed) with MultiMC
• Add Quark r2.4-271 and AutoRegLib 1.6-46 (I repeatedly checked to make sure these were the most recent versions. :P)
• Launch the game and generate a world (I was using a fixed seed foxthingfox for reproducibility at this point just in case.)

Expected behavior: the game loads normally
What I got instead: the game crashes either as soon as world generation is complete, or immediately (at best a few seconds) upon entering the world

It does work with Quark and AutoRegLib disabled. It also seems to work with AutoRegLib but without Quark.
[edited for a nasty word that showed up in the previous log because I was rather frustrated when making the instance, when I did this originally.]

I think I nailed this down to be a bug with Mega Caves and Crevices (they use the same code). Can you verify? Turning off both features should allow you to load the world.

I figured it out. Turns out adding water at this point of world generation is a terrible idea, so I just made it not do that.

A little late, but I can also verify that on my end, turning off Mega Caves and turning off Crevices (I had to edit the config to make it 'stick' for some reason) seems to allow me to generate, load and walk around in a world. Yay 🌸

Thanks for the fast reply! And all the effort you've clearly put into these mods over the years.