The bug
I added the Powersuits mod and Sophisticated Backpacks mod to my modpack and created a server for my community. One player crafted a backpack. Once he wore it on his back, all players on the server had their games crash, but the server was still fine. It doesn't crash everytime someone wears one because when everyone loaded back in it was fine. But then we all disconnected and rejoined the next day, and everyone's game crashed once that player joined wearing the backpack. After looking at the crash report, seems it has something to do with Powersuits assuming the backpack is "modular" when worn in the chestplate slot.

To Reproduce
Steps to reproduce the behavior:

1. Add Powersuits and Sophisticated Backpacks to modpack and launch server
2. Craft Backpack
3. Wear Backpack on character's back
4. It might crash everyone's client, but not the server
5. .See client crash report attached.

Crash Report
crash-2021-08-06_21.44.44-client.txt

Versions
Minecraft 1.16.5
Modular Powersuits 2.0.4
Sophisticated Backpacks 3.0.3.297
Forge 36.2.2

This is the section of code the mod author to Sophisticated Backpacks pointed out as causing the issue:
https://github.com/lehjr/MachineMusePowersuits/blob/1.16.5/src/main/java/com/github/lehjr/powersuits/client/event/RenderEventHandler.java#L109-L110

This should be fixed in the current version