The backpack randomly stopped opening mid-game. No clue why but I think I closed it and it just stopped working. I thought I'd taken it off accidentally, but the lava and water tanks are still in the corner of my screen. The backpack is on my back and I've tried deleting the mod and redownloading, changing the key for opening it and reloading my game, it still isn't working.

MC version 1.15.2 with mod version 4.0.8

That's probably because you exceeded data on itemstack, that issue sadly isn't fixed on 1.15.2 and so this version is no longer supported. Sorry for inconvinence