Forge version: forge-1.16.5-36.1.2
Twilight Forest version: 4.0.426 (1.16.5)
Link to crash log:
crash-2021-04-18_11.45.09-server.txt

Steps to reproduce:

1. go to the stonebrick castle (biome: twilightforest:firefly_forest)
2. place or destroy some blocks near or in the castle
3. the server will crash in a few seconds