Driver version?

Straight from AMD's release, driver versions after ~13.xx (it's a toss up for 12.8 and 13.1) will just throw a yellow exclamation mark in the device manager on videocards 5+ in Windows 7. Modified drivers take care of this issue. One of mine with that issue is running the same Asrock BTC mATX board using powered risers. 13.12 modified works fine though.

Not saying you're wrong, but I can't find enough details for that specific motherboard. It does share bandwidth between two of the x1 slots and the x4 slot (lower "x16" slot). When all three are populated, the "x16"/x4 slot runs at x1. So we have one x16 slot direct to the CPU, second x16 slot is running at x1, and two x1 slots are running at x1. Since PCIEX1_1 isn't mentioned in sharing bandwidth, it likely has its own lane. I think the Z68 motherboard should handle 5 videocards fine. I haven't looked into the other one. Therefore, I'd look at driver version.