If I come closer to a chest i suddenly can see it through the wall. Is this a bug?

Which version of Minecraft is this, and do you happen to be running any other mods that alter chests?

From what I can see it looks as though the chest is being rendered inverted... and I'm not sure of anything in Lootr that would cause this.

The problem is the mod "ModernUI" + "Optifine".

Fix: Change in client.toml of the ModernUI Mod "globalRenderer" to false.

Ah, thanks.