I assume you're doing a pool mining. Based on this thread it is because you send bad hash of good data or good hash of bad data.[1] You might want to share more details if you want to find the specific reason for your case, and should move this topic to mining section (either Bitcoin or Altcoins).
[1] https://bitcointalksearch.org/topic/pool-mining-why-are-some-shares-rejected-55991
Topic: Server Rejection Error (Read 122 times)
showing that we mined some blocks but server rejected with high-hashrate. Can some help why we get this error
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