This response is received when there is an intermittent connection issue in the telco's network.
First try redialing the ussd code. If the error response persists confirm that the sessions are received by the API. You can view this on your application dashboard. By following the below steps.
Go to USSD sessions
Select the three dots below the word actions
Check what app response or error message we are getting from your callback URL. That should help debug your code.
If you can't see the sessions then there might be a connectivity issue with the Tel-Co in which case we are following up with them.
If you can see the sessions then the issue might be with your callback URL. Here are some articles about the possible callback URL errors: http://help.africastalking.com/ussd#callback-url-responses
If none of these are of help kindly raise the issue with our support team at firstname.lastname@example.org