I have the same motherboard with 8gb of dual channel 2133mhz g-skill memory running an A10-7700k and it does work just fine so maybe it is the 7850k though they aren't all that different.
Are your enabling/disabling dual channel in bios or just removing a stick? (not sure if that's possible just asking) Maybe try playing with the memory profiles in the BIOS or other cpu/northbridge settings.
Benchmark-wise, the 7700k is pretty damn close to the 7850k so maybe you'd wanna just call it a day, return that one and try the 7700k. Either that or upgrade the motherboard.