Hmm... I spoke to the Dragon Princesses I and II. And when I am asked if I agree to cooperate with them, when I choose a positive answer, in both cases I receive a notification "You already promised to help another Princess". But I don't remember there being any other princesses to whom I made any promises of help at the moment. Or, maybe, I'm missing something?
Maybe there was another Princess I could accidentally make a deal with? Except the both Dragons?
Hmm... may be Skeleton then. I have quest "I will (not) marry you" in my journal. Does that mean I'm going with her now, so I can't follow the other princesses storyline?
I thought that this only applies to dragons, but it looks like it's much more variety... This is a good thing. But it would be nice if you actually warned about it.