The world used to have Immersive Tech, has been removed, started, multiblocks broken, saved. After that I added Immersive Technology, world crashes on load.
crash-2020-07-17_20.54.42-server.txt

When trying to load a backup that had not been "cleaned" of Immersive Tech the following crash occurs:
crash-2020-07-17_21.11.27-server.txt

@Kurtchekov that first crash is interesting with the CME.

The second crash is also interesting, when it tries to reform the blocks on conversion, error because of no hammer?

Yeah, player is null and therefore we can't get to see the hammer. @SableDrake when you tried removing the original IT first and replaced with this one, did you have the experimental pipes added by this mod enabled in your config file?

@Kurtchekov He did but I had him try disabling them. Same result either way.

It could be another mod causing that exception. Remember, that's a bug introduced by Forge, any mod can trigger it under the right circumstances.