Full log: https://gist.github.com/Juuxel/f45bde9f7f85766c8a24238fa97bf2d4
Full mod source: https://github.com/Juuxel/Adorn/tree/c034f0bdf49859f5bf9b1e42625aea3caabe8701

Transferring items into a SimpleFixedItemInv through a PartialInventoryFixedWrapper (custom subclass, link) crashes the game with an IllegalStateException.

It seems that hoppers modify the stored stacks in HopperBlockEntity.transfer:

I think trying to treat inv stacks in a FixedItemInv as immutable is a very bad idea. Mojang has made it very clear that mutable ItemStacks are the way to go, and LBA should follow that.

This should be fixed properly in 0.4.1 or 0.4.2 - I'm not sure which one fixed it properly, but either way it works correctly now.

@Boundarybreaker That really belongs is a separate issue, although I guess we're talking about it in the lba discord channel so we might not need to.