please help in identifying the root cause of NL-HTTPERROR-01 & NL-NETWORK-01 error code during Load test execution

While execution of 5k Virtual users, we are getting more number of errors in below error code,

• NL-HTTPERROR-01
• NL-NETWORK-01

Error detected by NeoLoad
Error Code: NL-HTTPERROR-01
Message: Internal error: An HTTP error occurred sending the request.
Details: org.eclipse.jetty.client.HttpResponseException: Received HttpResponse[HTTP/1.1 503 Service Unavailable]@4b313265 for HttpRequest[CONNECT / HTTP/1.1]@ab1b51c
Please refer to the documentation for further details and advice

Error detected by NeoLoad
Error Code: NL-NETWORK-01
Message: Network error: An IO error occurred sending the request.
Details: java.nio.channels.AsynchronousCloseException
Please refer to the documentation for further details and advice

Neoload Version: 6.7
No. of Vusers: 5k
Load Generators: 4 (located at Frankfurt)
Controller :1 (Located at Frankfurt)
Test: Accessed controller machine via RDP from INDIA.

Please help.

Thanks,
Mariappan Rajagopal