Looting a chest too often within a certain span of time can temporarily reduce the quality and amount of treasure that chest generates for you. This is to prevent players from farming the same chest over and over, especially chests that certain builds can reach quickly and chests that contain highly sought named items.
Every single chest in the game has its own chest ID and "ransack timer". Timers are tracked separately for each character. They are not tracked separately for each difficulty level (with a couple of exceptions).
The first time a chest is opened, it generates normal loot for you and the game starts a secret timer that lasts exactly 7 days (168 hours), right down to the minute. The second through seventh times you run that dungeon and open that chest, you also get normal loot. Starting after the 8th time, you get a warning notice, the chest's gold and ordinary loot levels drops by 1 (cumulatively!), and many named items stop appearing. You get no loot at all once you've driven the loot level to 0. How long this takes depends on the initial loot level. So, you can loot 8 times per 7 days without ransack. The 9th looting is the first to be reduced (and have no named loot).
The ransack timer counts down even through game updates and server outages. Once it runs out, that chest refreshes completely. It'll provide full loot and start the timer again the next time it's freshly opened.
[edit] Tip
Due to the way the ransack mechanic works, if you close and re-open a chest, it shows you the message you would get from your next looting, not the current one. So if you get no message on your first opening, but re-opening it gives you a ransack warning, you know you just got your 8th and last set of normal loot this period and your next run will be penalized unless you wait the timer out. This is an easy way to know if you will soon be ransacked without keeping track.
Opening a chest repeatedly in one quest does not advance your ransack count extra times. This has been confirmed by a developer, and tested (link ⇪).
En gros jusqu'au 7eme loot tout va bien c'est à partir du 8 eme que le loot diminue
Si qqun peut traduire ça serait cool