EWS API Error

Question

When I was using the EWS API to get exchange data, API frequently reported errors，like this:

"One or more errors occurred. The request failed. An error occurred while sending the request"

above error is from a piece of C# code

<<

var service = GetService(email);

var g = service.ExpandGroup(group_email).Result;

it some times run ok ,but reported errors frequently