Don't know why it would be stuck, one of the first things I did hitting floor 6 on my second playthrough was access this. As soon as I visited 7 proper and came back, I had access.
EDIT:
SpoilerShow
Just went through on a fresh save (just got to level 6). The door opened after I had been to level 7 and had the Sword of Nex. I didn't do anything special or uncover any other secrets.