Keys and chests

[fiery flower]

There are quests where chests are unlocked with keys. You need to obtain the keys by killing mobs. So, if you get the keys and open a few chests, but not all of them, and then restart the game, the chests you've already emptied become locked again and are highlighted with stars. If you try to inspect the chest's contents, you'll spend the key, even if the chest is already empty. If the number of mobs with keys equals the number of chests, you'll be unable to complete the quest.

[fiery flower]

This is what the chest I already opened looks like.
There are no stars. I was mistaken. But the chest is locked and requires a key.

The problem was resolved by restarting the game. A new mob with the key spawned after the game restart.