Sorry for the late responce, but if you haven't figured your problem out, I'll try and offer a solution.
If I'm not mistaken, the E38's have two crank shaft sensors. Not sure where the other one is located, but the code won't specify which one has failed, if I remember that right. Your suspicions about the crank sensor may be correct in this case.
At first instinct, it sounded like a MAF problem or false air problem. But since you checked that, it's probably the aforementioned crank sensor. Hope this helps.