We've had this crash on Daniel's latest Arclight Fork (1.0.25) running under his latest Velocity release 4~ times on a new heavily modded server. The server is running on Easy difficulty.

Here's the full crash report (https://pastebin.com/swY73myP), and here's a second crash report (https://pastebin.com/pcw2BBv3)

BetterSpawnerConfig hasn't been touched, here's the spawning.yml config: https://pastebin.com/VjJZsQjM
Here's the Bukkit.yml: https://pastebin.com/44zpxwsS


We've not experienced it on our standard Pixelmon SMP server running the same Arclight/Velocity. This is the modlist for the crash-y server:
SimpleStorageNetwork-1.16.5-1.5.3.jar
pamhc2trees-1.16.3-1.0.1.jar
pamhc2crops-1.16.3-1.0.2.jar
Chisel-MC1.16.5-2.0.1-alpha.4.jar
jei-1.16.5-7.7.1.152.jar
pamhc2foodextended-1.16.3-1.0.4.jar
flywheel-1.16-0.2.5.jar
FastLeafDecay-v25.2.jar
CosmeticArmorReworked-1.16.5-v5.jar
create-mc1.16.5_v0.3.2g.jar
pamhc2foodcore-1.16.3-1.0.2.jar
EnvyGTS-Forge-4.0.3-1.16.5.jar
journeymap-1.16.5-5.8.5p5.jar
CTM-MC1.16.1-1.1.2.6.jar
StorageDrawers-1.16.3-8.5.2.jar
Pixelmon-1.16.5-9.1.2-server.jar
byg-1.3.6.jar

We haven't found any common action causing the crash, it's very difficult to get an idea of what people were up to at the time.

Thanks, and if there's any more info needed please let me know.

This issue originates in Arclight, please refer to their tracker