I have the exact same Problem. My main and second main character can't start another dialogue anymore. This happened, when they both had the exclamation mark over they're heads in the secluded beach area. I saved before i wanted to undergo their dialogue. After the save was made, the exclamation marks were gone. And they could not enter any more dialogues. In all saved games... like with this person. So i think the saving mechanism triggered this problem in my case, with any further dialogues.
I hope you can find what is going on through my comment.
