One thing I discovered a few years back was that the TIP and RING conductors of my cheap USB sound card's jack are strapped together internally.
This means that if you insert a 2-conductor plug into the 3-conductor sound card jack, your scanner will 'see' a dead short.
If you are using home made patch cords, make sure there are no transpositions. Maintain TIP to TIP, RING to RING, and SLEEVE to SLEEVE on each end of the cord.