Update: Currently many of the Star Jam tracks on the website are in 48K format. These will not work and need to be converted to 44.1K before the Ditto X2 will import them. Kind of amazing this little point got missed, but it will certainly be a show stopper for anyone trying to use them. (TC electronic support has been notified and said this issue would be directed to the dev team.)
As for re-importing your own tracks, the only method I've ever been able to use is to convert the exported TRACK.WAV file into another 44.1K wav file (or .mp3). Yes, that sounds silly - but it works. I have never otherwise been able to successfully re-import a saved track.
I've written a blog post on how to resolve the import issues I have been experiencing: