It's the third or the fourth time I experience some trouble at the end of this quest.
I remember in the old versions of UOP v3.2.0 I never experienced these troubles and I played tens of time !!!
Quoted from: http://www.uesp.net/wiki/Oblivion:Caught_in_the_Hunt
* Prepare ahead of time by making sure that Aleron is on the lowest level of the ruins before you enter the Hunters Run.
* On the PS3 and Xbox 360, the only other option is to load from an earlier saved game. This game must be from before the last of Kurdan's three Hunters in the Hunters Run have been killed. Before killing the last hunter, be sure to head outside and push Aleron down to the lowest level of the ruins.
* On PC, you can use the Console to kill Aleron:
1. In the console, type: prid 0001FEDA
2. Type: kill (This will kill Aleron Loche, and may fix the quest. If not, continue to step 3.)
3. Type: prid 0001FDB7
4. Type: kill (This will kill Kurdan gro-Dragol as well. You should fight him, but if the game glitches on you, you may as well cheat just to get past it.)
5. Close console, and you should be good to go.
* PC Only This bug is fixed by the Unofficial Oblivion Patch.
# It is possible to kill Kurdan, search his body, retrieve the key, and open the outer gate but you never receive the next quest update; you are unable to finish the quest. This bug can be triggered if you decide to pickpocket the key off Kurdan before killing him, or if you kill him with an attack or a spell that makes him fly into the air and then search him before he hits the ground, or are too close to him and retrieve the key from his corpse before the quest update that appears when he's dead.
* PC users can use the Console and type setStage MS08 90 in order to get the next journal entry.
* PS3 and Xbox360 users need to wait for the journal update where you're asked to search his body for the real key. After this update you can search the body, take the key and you will receive the next update.
* PC Only This bug is fixed by the Unofficial Oblivion Patch.
It seems that these problem are coming back again to Oblivion although it is said they were solved in UOP (which was true).
Edit.
Note:
The only way I have found to not trigger this bug at the end is to not open your inventory after killing and looting the last pursuing hunter, until you get out of the dungeon (to confront Kurdan).