I imported my Witcher 2 save with no problems, but I didn't even know I had Witcher 2 savegames still on my drive.
My suggestion is to check that your Witcher 2 savegames aren't under another account on Windows, or if some other user permissions error is the cause. Try loading your Witcher 2 savegame in Witcher 2 to make sure the data isn't corrupt. Also, check if your Witcher 2 game is patched to the correct version for importing into Witcher 3 (I've no idea if that's even necessary, but it's possible).
Otherwise I'd take it up with the developers support.