Info:
Cause Of Crash: "fire = getEntityData().get(DATA).getBoolean("Fire");"
Caused By: entity.chemical_sprayer_projectile
Bug Finder: ziad25139 and Lazeryeet

Crash Report: https://mclo.gs/E1hEV4n (aternos)

Same bug happened to me: https://github.com/george8188625/Create-Diesel-Generators/blob/1.20.1/src/main/java/com/jesz/createdieselgenerators/entity/ChemicalSprayerProjectileEntity.java#L121

level() instanceof ClientLevel doesn't work on multiplayer, the class ClientLevel only exists on the client, not in the server. The correct way to detect the client side with level().isClientSide Ref: https://nekoyue.github.io/ForgeJavaDocs-NG/javadoc/1.18.2/net/minecraft/world/level/Level.html#isClientSide

Oh I'm stupid

I believe that's why the crash only applies to servers and not single player worlds which I tried testing on one

yup, dedicated servers don't have client classes