When you are in the nether loading into a chunk where a nether structure spawned can be any modded nether structure the entire world freezes and the game has to be closed with task manager to recreate:

1. create a new world
2. go to the nether
3. locate and teleport to a modded nether structure even the idas necromancer's spire can cause this
4. freeze
   Please note this is a 400 large modpack i am using so it might be a mod conflict not sure i can provide my modpack if needed here are the latest logs (and an rei log ) from the freeze

rei-issues.log

latest.log

rei.log