As far as I remember, Wordfast creates TMX files in either the system encoding or in UTF-8. Trados in turn uses UTF-16 (I'm not sure with which byte order). So, check the encoding of the TMX file and change if necessary.
I have solved my problem. The Target language code used is newer than in the Trados version. I just replaced languahe codes with "search+replace", and everything is fine now.