"Frida's Dilemma" - "Negotiations" [Campaign 1, Mission 4]

So, I guess I'm lucky. I completed all the campaigns of Heroes 5 and its expansions with only one bug in the elf campaign (Biara standing still in the first mission). Heh, maybe I should play it again. Sometimes these bugs appear, sometimes they don't.

Added 2 minutes later
You can also try a cheat to unlock the screen. But if Laslo hasn't appeared, it probably won't help.