Yep, I just got stuck exactly the same way. In one video walkthrough I found, the guy went to Kiev while his rebellion was still going on and it worked fine. I went to Kiev after we won (so Radziwill is already the king). I talked to mayor, he told me the long story, but now when I talk to Radziwill I cannot turn in "Kiev's mayor report" quest, nor can I ask him about Black Mace for the "Intractable Radziwill" quest that I picked up in Kiev.
Is there a "cheat" way to move a quest chain somehow?