java.lang.IllegalStateException: Entity is already tracked!

What the log looks like

java\.lang\.IllegalStateException: Entity is already tracked!
ReportedException: Adding entity to track
"Silently" catching entity tracking error\.

How to fix it

1. Check the stack trace for `TileEntityMobSpawner` or `MobSpawnerAbstract`; if present, find the custom mob spawner in the loaded area and break it or use an NBT editor to strip `UUID`, `UUIDMost`, and `UUIDLeast` tags from its `SpawnData` and `SpawnPotentials`.
2. If the error triggers when loading chunks (`ChunkProviderServer.loadChunk`), back up your world and inspect the region files around the player's coordinates with MCA Selector or NBTExplorer to delete duplicate entities.
3. Update chunk-loading or web-map plugins and mods (such as Dynmap) to the latest build for your Minecraft version so chunks and entities are never loaded off the main server thread.
4. If a Forge server crashes immediately when loading the chunk, back up the world and temporarily enable `removeErroringEntities` in `forge.cfg` (or `forge-server.toml`) to purge the conflicting entity on load, then set it back to `false`.

Check your own log