Long story short - pool problem, and only mining.blue fixed it till now. A bit longer story - that can happen because of not suitable intensity, or just because your GPU's found a share right before/after pool updated it scratchpad and send you new job. And because of different scratchpad's(your and pool) you can get reject. Anyway, this is not a big problem if you get it near "new job" message. If you get it in the middle of other "accepted" - this is another story and this is bad sign.