Hy,
i have the same error with DEM Coin. (eMark)
ThreadRPCServer method=submitblock
ERROR: CheckBlock() : first tx is not coinbase
ERROR: ProcessBlock() : CheckBlock FAILED
eg:
2017-10-19 17:44:10: REJECTED DEM block 1476596 - 0700000095d51a69d469778af82639da94e4d5df37a0ab1e83c2e59551000000000000007c13f4b5e5e5088ae8298f961b3521316c4ea26b1d3ef3eb0403228eadcfb3fe29c8e859fad7011a053a92010102000000010000000000000000000000000000000000000000000000000000000000000000ffffffff1803f48716042ac8e85908810000086e3c01007969696d70000000000002604ff302000000001976a9140830089232c684c379ad1cdd1ccbcb967c72c0ee88ac20a10700000000001976a91478565009c7de43288165d5b823eabdf11bd1424a88ac00000000