Need Help With Lost Sanctuary Dungeon

Hi there,
I just arrived at the Lost Sanctuary Dungeon located on the bridge between Time Valley and Elkurn. However, the entrance appears to be blocked by a giant boulder. Since I have completed all available missions up to combat level 29, there should be a Key Collector to let me inside. The key that's necessary for this dungeon is also in my possession.
How do I fix this?

EDIT: The dungeon randomly opened after I returned back to the game. Still wondering why and looking for an answer in case I want to do the dungeon again.

EDIT: Alright, so I believe I found out how this works. Three NPCs should appear once you enter the area of the Lost Sanctuary dungeon. Those NPCs were part of an earlier mission and should destroy the boulder to open the way for you to enter the dungeon. However, if you wait too long inside of the dungeon area, the boulder will respawn and you will have to try and re-enter the area to remove the boulder.