minecraft_version=1.21
yarn_mappings=1.21+build.9
@loader_version=0.15.11

Fabric API
fabric_version=0.100.7+1.21

Setps To Reproduce My Issue :
Setp 1 : Make a Custom Potion Mod in Fabric for the latest version (1.21).
Step 2 : Register the potion following fabric docs .
Setp 3 : Brewing The Potion In a single player world which should lead to [Server thread/ERROR] Failed to save chunk -1,-2.
Step 4: Break The Brewing Stand Block with the custom potion in it to crash both server and client.
Referance :
Discord - 1
Discord - 2
Crash Log

It appears you are constructing new instance of RegistryEntry; you should be looking them up from Registries.POTION instead,