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Reloading didn't help. I tried that right away. I'll try replaying the mission.
It happened when I created my third hero, to deliver an army to the main hero. I don't remember exactly, but I probably had one hero, or maybe even two, in my main city.
Added after 4 hours 8 minutes
I'm playing through the mission again. This bug isn't present yet, but another one has appeared. When I go through the waterfalls for the first time towards the gate, the view of the gate opens. There's a hero standing in the gate. From below, another hero, disguised as a snake, is approaching. Last time, this snake went up, and the gate closed. This time, it goes to the right in front of the gate, and the game freezes. The only way out is through Ctrl+Alt+Del.
Added after 4 hours 43 minutes
I replayed it twice. The same bug. It freezes in the same place.
Added after 55 minutes
Looks like I'm just talking to myself.
Anyway, the problem is solved. I started taking other cities and waited for the hero from the gate to finally come out. And then everything went smoothly. If it weren't for the bugs, the mission would be quite simple.