This occurs when IOMMU is on and Base Mosaic is in use.
The primary card is on 03:00.0
The driver does not map read permissions in the IOMMU for the secondary cards.
Driver 340.104
Linux 4.14 64-bites
Quadro NVS 420
Looking through the Linux source, what would seem to be a solution to this is to have the driver add all the detected cards into the same iommu domain before initializing base mosaic if iommu is enable on the system.