Déjà Vu X2 v8.0.519 appends the text utf-8 to .sdlxliff file headers when exporting. In some cases, the outcome of doing this is that the codepage label in the exported file header, initially utf-8, becomes utf-8utf-8. Trados Studio is unable to parse this construction and thus it can't read the file.
The latest version of Déjà Vu X2 does not have this problem. You are, therefore, advised to update to it.