When loading into the Nether the game freezes and this comes up in the log

[Server thread/WARN] [net.minecraft.world.server.ServerWorld/]: Trying to add entity with duplicated UUID 706b98e8-966c-464f-a724-0f650264a6c4. Existing rats:demon_rat#10, new: rats:demon_rat#10
latest.log

I don't know if this is a mod interaction or not, but removing Rats fixes the issue

see #671