I ran into this as well. Could have sworn I had a bethesda fourm account from years ago, but can't figure it out. Anyway, luckily I save my saves. Last workable is over 8 hours of play ago though, so hope I don't have to go back to that.
At 21.08 playtime I am right about to use the elevator to do the Curtain Call mission. I can access the Plaza.
At 21.21 playtime I am on the top part of the tower, about to fight the boss there. Can't access the Plaza.
All saves before 21.08 Can access the plaza, all saves after 21.21 can not. So I would think there's something in Trinity Tower is mucking up my save there.
I've now got the files on mediafire both as individual saves and both in zip. If anyone wishes to have them, I'll send the link via PM. As Gstaff instructed not to post the link on fourm proper.