Ich hab in der UESP folgendes dazu gefunden:

The Countess's jewelry box can contain two identical copies of the quest ring. Immediately after you pick up the first ring, you will get a quest advancement telling you to deliver it to Ahdarji. As for the second ring, you must leave it behind. If you pick up both rings, Ahdarji's dialogue options will no longer include the "I found it, here you go" option, and you will be unable to complete the quest. This means no more Thieves' Guild missions. Also, both rings are quest items that cannot be dropped or sold, so you'd be stuck with them forever.
  • On the PC version, there is a workaround: use the console command player.removeitem 00035E95 1 twice until the counter says 0.00, then player.additem 00035E95 1 to fix the problem.
  • Otherwise, you can murder a NPC in the castle where you collected the rings and a pop up will inform you that you must pay a blood price. Pay your blood price, and you will finish the quest and be able to complete the other quests.
  • This bug is fixed by the Unofficial Oblivion Patch.

Versuch mal das fett markierte. Falls dir das nicht hilft, durchsuch mal die anderen möglichen Bugs, ob dein Fall dabei ist.