Firstly I would strongly recommend setting the address on the programming track first. If there is a short, you risk blowing the decoder if you place it straight on the track at full DCC voltage.
As Richard says, you could first check that the pins of the decoder are not touching the metal underneath. Try putting some insulation tape underneath, or easing the decoder up a little so it's not pushed fully home.
To run a loco on DC, a blanking plug simply joins pins 1 and 8 at one end, and 4 and 5 at the other. Due to faulty manufacture, one of these pairs of pins could be connected underneath by a blob of solder. However this will have no impact on DC as that's all the blanking plug does anyway. But it will cause problems on DCC when you fit a decoder. If you have a meter, you could try removing the plug and testing that these pins aren't electrically live to each other. If they are, it means you'll have to get underneath the PCB and break the false connection with a small file etc.