Daemon instance 0 rejected a supposedly valid block

[devAji917]

i have forked new coin. coin is pow/pos masternode. i have changed genesis info also.
when i mining pool says that as follows:
any help are welcome.
please help me.

2018-04-10 17:17:40 [Pool] # [cropcoin] (Thread 3) Daemon instance 0 rejected a supposedly valid block
2018-04-10 17:17:48 [Pool] [cropcoin] (Thread 2) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:17:48 [Pool] [cropcoin] (Thread 1) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:17:49 [Pool] [cropcoin] (Thread 4) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:17:49 [Pool] [cropcoin] (Thread 3) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:17:53 [Payments] [cropcoin] Finished interval - time spent: 3ms total, 1ms redis, 2ms daemon RPC
2018-04-10 17:18:13 [Payments] [cropcoin] Finished interval - time spent: 3ms total, 2ms redis, 2ms daemon RPC
2018-04-10 17:18:33 [Payments] [cropcoin] Finished interval - time spent: 4ms total, 2ms redis, 3ms daemon RPC
2018-04-10 17:18:43 [Pool] [cropcoin] (Thread 2) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:18:43 [Pool] [cropcoin] (Thread 1) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:18:44 [Pool] [cropcoin] (Thread 4) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:18:44 [Pool] [cropcoin] (Thread 3) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:18:53 [Payments] [cropcoin] Finished interval - time spent: 3ms total, 2ms redis, 2ms daemon RPC
2018-04-10 17:19:05 [Pool] [cropcoin] (Thread 3) Daemon instance 0 rejected a supposedly valid block
2018-04-10 17:19:13 [Payments] [cropcoin] Finished interval - time spent: 4ms total, 2ms redis, 3ms daemon RPC
2018-04-10 17:19:33 [Payments] [cropcoin] Finished interval - time spent: 3ms total, 1ms redis, 2ms daemon RPC
2018-04-10 17:19:38 [Pool] [cropcoin] (Thread 2) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:19:38 [Pool] [cropcoin] (Thread 1) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:19:39 [Pool] [cropcoin] (Thread 4) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:19:39 [Pool] [cropcoin] (Thread 3) No new blocks for 55 seconds - updating transactions & rebroadcasting work
2018-04-10 17:19:39 [Pool] [cropcoin] (Thread 3) Difficulty update to diff 8 workerName="QVzFQcFgm7K6uSiWM2ZFTtF7z91qdBhSQ9"