Hi, all 3 pci-e connectors were plugged in and both hashboards were operational until recently. One interesting thing is the data socket and cable was also burnt although not as bad.. so what ever happened also happened to the data cable.
From the view of an Engineer well versed in power systems, seeing just that one PCIe connector burnt is highly suspicious.
Either:
A) The board was powered by just that one connector. If so, of course it fried...
B) Very bad sockets in the PCIe cable feeding it. I doo see that there is no solder fillet around the pins where they join the board. I will assume that there is good solder on the backside. Lack of a bead on top is not good but not conclusive.
One problem with "B" -- if all 3 connectors were being used, they should have picked up the slack resulting from the poor connections. Remember: All 3 of the PCIe connectors are in parallel. In no way could there have been enough current through them (bad connections) to burn it and the 2 good connectors show no signs of overload at all rg, turning brown.