Error when Check Answer of code test

[thaihaquang]

Dear authors,
Thanks for your project.
I am running your service for testing code. However, when the student clicks the "Check Answer", the pop-up shows "There is some problem. Try later."

• Error information from Client Browser "POST http://192.168.0.104:34567/exam/721/check/1/2/62/55/ 500 (Internal Server Error) - jquery-3.3.1.min.js:2"
• Error information from Server log
  requests.exceptions.ConnectionError: HTTPConnectionPool(host='localhost', port=55555): Max retries exceeded with url: / (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f690c24e358>: Failed to establish a new connection: [Errno 111] Connection refused',))
  [18/Jun/2021 08:19:47] "POST /exam/721/check/1/2/62/55/ HTTP/1.1" 500 217193

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I run the service by "invoke serve"
Please help me to check it. Thank you very much for supporting.