Фармазон
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Have you tried starting the 4th mission again?
I tried, I even started the whole campaign again, it doesn't help...
Added 5 minutes ago
Turns out I'm lucky. I completed all the Heroes 5 campaigns and expansions with only one bug in the elf campaign (Biara standing still in the first mission). Heh, it's worth trying again. Sometimes these bugs occur, sometimes they don't.
Added 2 minutes ago
You can also try a cheat to unlock the screen. But if Laslo doesn't appear, it probably won't help.
It happened to me too, but replaying helped, but here...
The thing is, Laslo appears, but the dwarf doesn't!!!!