Does the problem occur if you run the rig without that card (ie as a six card rig instead of a 7)?
Have you tried swapping cards in the risers to see if it still happens in that position or does the problem follow the card?

The same thing here. I got all rigs except 1 working with v. 2.1-3.1 with -i 2. The problematic rig has 6 R9 390 which work about 20 min with 60-ish sols (and over 1% rejected shares) then 1 card stops. Curiously, with -i 0 there is no problem and the cards give 70-75 sols! I tried everything but it is not the card nor the riser.

my 7 card rig is always tricky, are we too ambitious? which mobo you using?

I am getting the same issue as everyone here. Every rig is fine except one 6 GPU rig with a few 290s.
It hangs and you cant close the program , need to reboot.
I think the issue is that with regular mining, the GPU used little bandwidth to communicate with the CPU so the cheap ribbon or usb risers were fine.
But now with huge 100-200MB/s bandwidth going thru the ribbons which probably have interference with the surroundings is causing problems.
Try running with -i 0