I'm playing a modded world featuring mainly Dragon Survival, Sophisticated Backpacks, Sophisticated Storage and Iron's Spells and Spellbooks. Everything was going normal for days, I have a double dark oak chest with iron upgrades that I'm storing all my stuff in, but I added a starter tier stack upgrade to the chest, sorted it, and then my game crashed. I can load back into the game fine and everything plays normally, unless I open that chest, then it crashes again.
Minecraft 1.21.1
NeoForge 21.1.206
Sophsticated Core 1.3.68.1116
Sophisticated Storage 1.5.1.1260
Sophisticated Backpacks 3.24.22.1319

latest.log

stackableitems is the cause - looks like they are trying to reformat the stack count and just parse it as number when it doesn't necessarily need to be just a number when higher counts get involved and something like 1.1K is used instead of 1,123. They need to fix this on their side.