1.18.1
Forge 39.0.59
9.0.1-79

I am experiencing existing worlds crashing on world load. New world load fine, and I am able to load existing worlds when I remove modular routers.
crash-2022-01-23_12.04.48-server.txt

Yeah, current release isn't compatible with current Forge (it's a fast moving target right now). Next release will be. Out in a day or two, hopefully.

Fixed in 9.0.2 release