Steps to Reproduce the Bug

On Fabric 1.20.1, my game crashes when joining both the existing and the new world.
With spawnermod-1.20.1-1.2.4+Fabric I had no such problem.

Enhanced Mob Spawner filename

spawnermod-1.20.4-1.2.5+Fabric

Other mod filename (Optional)

No response

Extra Notes (Optional)

Crash report and latest.log:
https://gist.github.com/mpustovoi/deab958d379a19105c6238812656c820

Thanks for the issue.

I realized that I had ticked the latest 1.20.4-1.2.5 version to be compatible with lower 1.20.X versions - which is not true. So please downgrade to your previous version: spawnermod-1.20.1-1.2.4+Fabric.