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The issue is with the patch. I replayed several times on V2.0 and encountered a bug, but after installing V2.1 and replaying, everything was fine. Therefore, I recommend installing the patch.
In short, I figured it out myself. I replayed several times over the weekend and got tired of it. As it turned out, this bug occurs if you kill the zealots guarding the exit through which the prisoner will later leave, BEFORE freeing the prisoner. At least, that's what happened to me. I played without the patch.