Normal Crypt of Hearts, the gate to the final area containing the final miniboss and final boss?
This happens if someone in your group has the dungeon story quest. They need to complete the steps in that quest (breaking the phylacteries) before the gate can open.
If they are unable to complete the quest steps (e.g., if the phylacteries are broken out of order, thus getting the story quest stuck), then your only option is for that person to abandon the story quest.
Once the story quest objectives have been met, or the story quest has been abandoned, the gate will open.