When you equip the backpack, the server will crash upon opening your inventory.

Here's log.

crash-2020-03-28_19.35.37-server.txt

Happening for me too.

Forge Version:

• 31.1.0
• 31.1.30
  (Tried both)

Minecraft Version:

• 1.15.2

Mods List:

• AppleSkin (AppleSkin-mc1.15.2-forge.1.0.13)
• AutoRegLib (AutoRegLib-1.5-40)
• Just Enough Items (JEI) (jei-1.15.2-6.0.0.2)
• Quark (Quark-r2.1-225)
• Quark Oddities (QuarkOddities-1.15.2)

Log File:

• https://pastebin.com/QvDXzBms
  (Crash occurs between like 253-293)

Quark 226 fixes it.