Forge version:
Twilight Forest version: 1.16.5-4.0.435

Steps to reproduce:

1. Loading the twilight swamp (dark forest area) with the RapidLeafDecay mod

What happened instead: Game crashes every time. I checked multiple times, RapidLeafDecay mod was the issue. I have not tried a different quick-leaf decay mod yet. Rest of Twilight Forest seems fine.

Send a crashlog so we can check it ourselves

I have a bunch of mods and it kept crashing until I removed RapidLeafDecay yesterday. However, I tried to recreate the issue today, I wanted to give you a version with just TF and RLD, but it wouldn't crash even with all the mods installed, including RLD. So below are a couple of logs from yesterday as I was testing, looks like an issue with Dark Leaves block, but who knows since it isn't crashing anymore. Hope that helps.

crash-2021-05-16_21.31.01-server.txt
crash-2021-05-16_21.21.48-server.txt

Your log shows you're using Rapid Leaf Decay 1.0.0 which is outdated, Version 2.0.0 shows it fixed this issue. Update the mod.

Actually going to close this since it's solved.
Specifically by this commit GeheimagentNr1/RapidLeafDecay@231af32