With a new cable, not the one included, it seemed to work again for a while. There's two connected, to two different USB buses, but i'll look into the the powered usb advise too.
After a while however, i got this:
[2015-01-11 20:59:14] AMU0: invalid nonce - HW error
[2015-01-11 21:15:01] AMU0: invalid nonce - HW error
[2015-01-11 22:12:15] AMU 7 SendWork usb write err:(-1) LIBUSB_ERROR_IO
[2015-01-11 22:12:15] AMU 7 attempted reset got err:(0) LIBUSB_SUCCESS
Then the now troublesome unit zombied. Got it up again briefly after some reset/un/re-plugs. Then it just zombied again.
AMU ? It doesnt seem to be detecting the U3 properly. It should be saying AU3. Are you using cgminer 4.9.0 with out any settings to test. Also the USB should be as short as possible, long ones seem to have issues.
I have 2 running with 2 short USB cables that I bought seperatly. I had the same errors as yours with the stock cable and then some other longer cable. I suspect its voltage related.