Which modpack version are you using?

2.1.0.0

If in multiplayer; On which server does this happen?

N/A This is SP

What did you try to do, and what did you expect to happen?

exited my world with 8 furnaces smelting magnetite and 2/3 smelting chalcopyrite. I expected to be able to load back into my world.

What happened instead? (Attach screenshots if needed)

A concurrency modification exception crashed my game.
https://paste.ee/p/WHmYc

What do you suggest instead/what changes do you propose?

The bandaid fix for me was downgrading to an NEI with 0 concurrency, letting my furnances finish smelting and then restart. This seems to have fixed all problems but still means there is a weird with concurrent modication in NEI.

What is your GTNH Discord username?

FluxDZ#3861