I figured out why crash happens. When your future wife does something extraordinary on the map (plundering the village, fighting on the battlefield against enemies, being in the prison, besieging castle as a part of the army, which means that she is currently unavailable at the moment of marriage bartering) the game will crash 100% percent.
As to me, I tried to marry Liena, I came to her farther to do final arrangements, but the game constantly was crashing, because she was captured by Empire and was in the prison. I waited some time, searching in encyclopedia her location. When I saw that she escaped from dungeon, I found her father immediately and successfully conclude marriage agreement.
