Version Information
Minecraft: 1.12.2
Forge: 1.12.2-14.23.5.2847
Iron Chests: 1.12.2-7.0.72.847

Report Information

Opening an upgraded shulker box (iron, gold or diamond) sometimes makes the player inventory disappear, including equipped items. Relogging makes the items reappear again, as does opening the Baubles menu. Despite this, while the items are "disappeared" their effects no longer apply (for example: the Boots of the Traveller no longer provide a speed buff), so they are really gone, and it's not just a visual glitch.

Steps to Reproduce

Place an upgraded shulker box in the world, and open it (only work sometimes). It's possible this is some sort of mod conflict, possibly with Quark, since I noticed that the inventory preview when mousing over an upgraded shulker box in your inventory still only shows 27 slots. This may be unrelated, however.

Crash Log

It's called a "crash log" in the document, but it didn't actually crash:
Shulker Box Crash Log.txt

Forge Log

No idea what this is, or how to find it.

While this glitch is somewhat inconsistent, it's still fairly easy for me to repeat, should additional information be required.

I am also having this issue and have a few things to add.

1: opening baubles menu, as stated above, does not actually fully fix the issue. While your inventory will reappear, attempting to move items around it or switch items to your offhand will not work properly, also you cannot use some items (eg, satchels from cofh cannot be opened)

2: I created a new world without quark installed and it did not resolve the issue, so that mod is not the cause. The world I created was an RLcraft one, which is also the base modpack for my original mod, so if this bug is due to a mod conflict it is one of the RLcraft mods.

3: I find the fastest way to reproduce this is to place an upgraded shulker box while wearing armour and having items in your quickbar and spam open and close on it half a dozen times. then mine the box, move somewhere else, put it down, and repeat until the effect occurs. Often the shulker box with stagger and not open right away when it finally occurs but either way you should see your quickbar empty or yoru armour vanish.