I've managed to import the save. The error occurs if the folder in C:\users\ with your username contains diacritics (in my case, Russian letters). I've created another plain-named user, imported the save after launching the game on that user, and moving the gamesaves folder back to the original user's Documents folder. It works.
I'm quite sure its not just user name, but whole file path containing cyrilic characters it cant handle. I suspected as much, but W2 handling it without issue thrown me off.
Tsk-tsk-tsk, CDProject, I expected better of you.
Many thanks, Detero.
