Game crashed when the backpack block was loaded.
Happening in Debug World or when I place the backpack with /setblock command.
Game doesn't crash when I place the backpack manualy from my inventory with Shift Click.

As the crash doen't occur in normal game/survival, it's not a big game-breaker, and thus isn't urgent, but it is a bug that should be fixed.
I presume that this would happen if the backpack was generated maybe with a structure, but I don't know how to do that, so I cannot test this scenario.

Crash Log (https://pastebin.com/aKjYU1uW)

My take is that it's trying to set an owner or something, and because it was generated it doen's have one. I don't know how this mod works, though, so I cound be completely wrong.

Thank you for any responses ☺

Edit:
I saw something in the logs mentioning Immersive Engineering, so I tested it without the mod. I have hoped it would be just a conflict, but the game is still crashing.