voici la soluce en anglais
Keys of Cavatum Quest:
The very first thing you do after the Abbot gives you the two quests is to go over to where the Keys of Cavatum were and look on the floor by the structure. You´ll see a wierd patch, which you should click to discover that it´s ´Crime Scene Grit´. Go back and talk to the Abbot, and he´ll give you access to the Levant.
Just keep going down, and talking to everyone you see. Eventually you´ll find Norman Malloy, who´ll tell you that Sister Angela is a suspect. You can go back to the tower at this point, but you´re best off completing the Sluice Gates Quest while you´re down here ( see below).
Assuming you´re smart and you´ve completed the Sluice Gates quest, head on back up to the cathedral. Talk to the robot at the top of the elevator, and he´ll gripe about the human working over by the cargo container that brought you up here. Go talk to the guy, and click on the patch of stuff on the ground by him ( "Tunnel Transport Grit"). Now go find the Abbot, who´ll direct you to go find Sister Angela.
The following small section requires ´finding´ a lot of people in a row, so use the Tower Guides ( the dudes with the spikes on their heads) and they´ll tell you where to go. Talking to Sister Angela leads you to find Brother Liseria, then Thomas Litton, the Brother Flammarion ( kneeling by the doors to the 9 Enigmas chamber), then Restlin Brewster. Then you´ll have to go find Stiletto, so use the exit by Thomas Litton, find her, then go all the way back to Sister Angela´s room. You´ll control Stiletto to search the room, but don´t worry if you don´t have any luck since I think the game just gives you what you need eventually. Then you need to go back to the Entertainment District, to the Eco Bar. This isn´t too tough... use the exit by Thomas Litton again, keep going straight and right, and eventually you´ll see the sign on your right. If you´ve reached the diner with the tables and umbrellas, you´ve gone too far. Inside, on the far left, is Felicia. She´ll tell you to go to room 3 at Le Sommeil ( second floor). Go there, lockpick the doors, find keys.