Please help, I'm having the same trouble. 100% rejects regardless of the difficulty port I use. Using sgminer 4.1 (within sph-sgminer), with the diamond.cl file replacing groestlcoin.cl in kernel directory.
Using ethOS, 5x 280x, dmd.miningfield as pool, etc. Have tried other pools, same result.
Please let me know if you need any further information. My hashrates look good and normal, btw.
Also worth mentioning, unlike the original poster, I am having this issue whether connecting to stratum or non-stratum pools.
I can't mine DMD until this is resolved, so any offer of assistance hugely appreciated. Maybe more miners will come around if you help me? Can't imagine this is unique, all I did was follow the directions afaik. Diamonds last forever.