Hallo there.

I do not detect errors on the console when I am in the game and using a backpack. But when I start the server or restart the server, this error occurs on the console. I have no idea why.

I don't use any other mods other than:
Goblins and Dungeons Mod
Bountiful Mod

Screenshot:

SophisticatedBackpacks version: 1.16.5-3.12.0.407
Forge version: 1.16.5-36.2.20

fixed in the latest release