After completeing the campaign, I got allied to the Vex, and as I was about to select some of the activities in contested areas, they literally just... vanished. In front of my eyes, the missions vanished. I cant select anything the only thing I see is the Tharsis Outpost social space.
I can't invade, or do anything else.
WTF?!
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I had same problem, fixed by pledging to same faction on second character that had finished campaign; game seemed to confuse progression on one character with another.