Forge version: forge-1.12.2-14.23.5.2847-universal
Twilight Forest version: (twilightforest-1.12.2-3.10.1013)
Link to crash log: https://gist.github.com/Adam-12100/5325351bbcd6e9411d6ba1698b8540d9

I tried to read the crash report but I don't really understand.

Steps to reproduce:

1. When someone tries to enter the portal

What I expected to happen:
To explore the twilight forest

What happened instead:
The server crashes

Thank you

The server was stopped because it fell too far behind.

You can change this behaviour with max-tick-time in your server.properties file. I would also check the logs to see if there are warnings about cascading worldgen.

See also #589, #371 and probably other similar issues.