I have the same issue.
First when i started this mission it was fine, but that time my power was around 50-60, and the mission needed 190 power.
so i came to main story pledged few places in maps gains some power and went back to complete it. now sometimes it wont allow to enter the cave.
if i fast travel someplace and comeback or if i do any other game task and come to cave and enter, it will allow me but the cave is now a glitch, its transparent and i can not do anything else than exiting the cave.

loading a save point is not a option here, as i will lose a lot of progress