Minecraft Version

1.21.1

Immersive Engineering Version

12.4.2-194

How did it happen?

Was trying to open a world in ATM10 To The Sky.

Crash Log

https://gnomebot.dev/paste/mclogs/0UlZQVM#L7819

Does this happen every time you open that world, or did it only happen once?

Happened only once.

Likely a matter of multithreaded loading causing an issue there. Gonna close this for now. Worth further investigation if we get more reports.