Here's something from the dim recesses of my brain that I'd just remembered. Blackmagic VideoHub routers take all their network traffic over port 9990; which is also used in some QLogic/SAN configurations for metadata traffic. They don't play nice with each other.
To fix this set the BM Software (whilst USB-connected to the hub) to use another port. Then you'll have to set all the client copies to use whatever number you chose. The default won't do!