Also, kind of a spoiler, but an answer to Werner's question:
Check what the sephiroth that you are supposed to find. If the one you are looking for is the Maal'kaad, you picked that one off of the cockatrice's body in the beggining of the chapter. I'm guessing this is the one that you are looking for because when I did the quest I didn't have to go into the Lionhead crypt.
If you allowed that Siegfreid guy in the sewers at the beginning of the chapter follow you along and he lived, when you first approach the guards at the dike entrance a cutscene should start with Siegfried walking in halfway through telling them that you can pass. If you didn't I'd suggest talking to him at the Order Outpost, right by the Order armorer. Also, if you haven't done already, talk through all Vincent Meis's options to see if you possibly missed something. Hope this helps.