Minecraft version - 1.20.1
Mod loader - Forge 47.2.20
Mod version - 1.8.4
I am using the latest version available - latest as of mc1.20.1

Describe the bug
Trying to join a specific world, results in a crash. some other worlds are fine.

To Reproduce
Not entirely sure, but:

1. Download Better Combat and Just Leveling
2. maximize all levels in just leveling and enable all abilities
3. leave and then rejoin the world
4. See error

Expected behavior
Joining the world without any crashes.

Screenshots or video recordings
crashlog: https://pastebin.com/NpCNdGcD

I'm having the exact same issue with both mods.

JustLeveling is causing the crash.
It is trying to modify the code of Better Combat, which is unsafe, and it fails doing so.
Remove JustLeveling.