WIN-10 :
same issue :

7 -> Could not connect to http://127.0.0.1:8888

JSON-RPC problem. Probably couldn't connect. Retrying in 1...
miner  10:46:04|main  Getting work package...
  X  10:46:06|main  Failed to submit hashrate.
  X  10:46:06|main  Dynamic exception type: class jsonrpc::JsonRpcException
std::exception::what: Exception -32003 : Client connector error: libcurl error: 7 -> Could not connect to http://127.0.0.1:8888 etc......

SOLVED :
open the rpc 8888 port ( windows firewall )

- start gkr with : gkr --rpc --mine "your_wallet" --miningthreads "your_threads"
- run Start_Mining.bat file (wait fro DAG file) --- done!