A mod is registering content at the wrong moment
Applies to Forge, NeoForge.
What causes it
A mod tried to register game content after the registry was locked, or two mods fought over the same entries. This is a mod bug or a loader version mismatch.
How to fix it
Update the mod named in the stack trace and the mod loader. If it is unmaintained, remove it.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log