This is a new but known problem with MU. See: [Only registered and activated users can see links. ] Post #87 and others earlier.
Use a fake nuvi 'stunt-double' of your device by putting the current GarminDevice.xml in a Garmin folder on a suitably large card or USB stick. GarminExpress will recognise it as the unit and you can dl all the required files then transfer them to the nuvi without having directly exposed it to GE.