Anubis,
Are you seeing the Imp Totem Shamans in the cave?
If so, one of them should be dropping a key that lets you through that door.
If not, then welcome to the Imp's Lair Bug Club. You'll need to email your Save game file to support@larian.com. They will fix the problem and email the file back to you. It took them less than 24 hours when I sent in my file.
Good luck.
