There seems to be a connection between the data line and ground. This may occur along the cable or the at the USB 1-Wire adapter. Verify the iButton holder is free of debris in addition for checking that the message doesn't appear when the USB 1-Wire adapter is connected to the computer without a cable attached.
Finally, one of the USB 1-Wire adapters, the DS9490B#, holds a single iButton device that can be inserted upside down. Although it is harder to insert the device upside-down it is possible. This will create a short as well.