It happens to me a lot.
Fix it by open the mission arc file in a plain text editor, and sort the objectives in the trigger order. I don't think exact trigger order is necessary, perhaps any objective resorting solves the false positive.
I discovered the solution when I just needed to sort the objectives to make editing more convenient.
Story arcs: �Viva la Represi�n! (#2327) | Mysterious Virus (#11762) | Four Horsemen of the 2012 Apocalypse (#531874) | Deadly Dragon | White Slime
My story on union.virtueverse.com
Ok, thanks for the advice, I'll try it out 
Yet another annoyance in my arc.
I have an ally triggered when three other allies are released. The Error box keeps telling me I'm creating a circular dependancy when the second ally should be triggered. Can anyone give me some help?