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We resolved Unre@lll`a's problem in a private message; we just didn't post about it in this thread.
For everyone who encounters this issue, removing Russian characters from the game's path and from the map's save file name in the editor helped. In other words, there shouldn't be situations like c:\Игры\Heroes5....
P.S. I left this question on the Nival forum in this thread: http://forum.nival.com/rus//topic/35039/. Several other solutions are also provided in another thread: http://forum.nival.com/rus//topic/34642/.
Тьма - это то, что позволяет смертным не забывать, как ярок бывает свет.