Bitmain Equihash miners do not work properly with nicehash. This is something I worked to correct but never completed the work as it is a little complex (the entire share has to be properly recalculated and submitted) to fix. The mistake is bitmain's, coupled with Nicehash servers responding oddly. For a given submission, some NH servers will sometimes accept it, and sometimes they won't.
It is beyond frustrating and to be honest, NH doesn't really seem to know how to fix things. I get the feeling that NH really is just a company that provides a wrapper service around everyone else's work (miners) and does not really understand stratum protocol, or share calculations... or it could be the sample set of engineers I was working with...
-j