I slid the riser in from the GPU slot towards the motherboard area, as it wouldn't go in the other way. Only after sliding it in did I plug the riser to the motherboard's PCIe slot.I'm not 100% sure what could be going on, but I'll give my best attempt to help.
How did you insert the PCIe riser? sliding it from the GPU area to the CPU area and plugging in the riser into the PCIe slot; or plugging the riser into the PCIe slot and pushing it into the GPU area?
Do the cards work if you plug them into the motherboard without the PCIe riser?
I'm a little worried about the loud popping sound that occurred.
Neither of my graphics cards work on this motherboard now, with or without the riser.
However both cards work in my other system, which is in a Silverstone RVZ02, which has its own riser implementation as well.
Thanks for sharing your thoughts and offering to help, I'm truly at a loss here.