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you better ask about this on the alsa and ultimately jack-devel mail lists, or #jack channel on irc.freenode.net.

but, could it be that that usb device is a "capture-only" one? that is, you can only record audio from it, no playback is allowed nor possible?--you may try then to set Audio := "Capture Only" on qjackctl/Setup/Settings dialog and try whether jackd starts in better shape that way :)

hth.
salut