Argggh. Got new iPhone 6 and iPad Air 2 with iOS 8.3 and lost my beloved Marvin / Calibre Connector!
Apparently iOS 8.3 breaks file transfer utilities:
http://www.macrumors.com/2015/04/13/...ox-app-access/
(as DopedAngel pointed out)
I'm guessing that is the problem. I also have a problem with the music manager MediaMonkey on iOS 8.3.
Have downgraded my devices to 8.2 while I still can and can confirm that the Marvin / Calibre Connector works again.