Sometimes, when adding a piece to the matrix, this crash happens. VanillaFix catches the crash and prevents a CTD, but the piece is never added either.

Once it happens, it keeps happening until I try to enchant a different item (and thus change what piece would be added next), which leads me to believe that this is an issue with a specific enchantment rather than a general problem when adding new pieces.

So far, I've only observed this to happen with books and swords.

https://paste.dimdev.org/ikevaqacax.mccrash

The pack is a heavily modified Crucial instance, and only played in single-player.

Thank you for your time ^ ^

Seems like a weird cross mod interaction. I recommend finding out what mods add enchantments and if removing any of them in a test instance would fix this.