Thanks for your replies. We can start to log the error code (as described here https://learn.microsoft.com/en-us/graph/errors?view=graph-rest-1.0#code-property) from the response body as @CarlZhao-MSFT suggests.
@Gopinath Chennamadhavuni :
Error 401 is an Unauthorized error. Make sure that your application is presenting a valid access token to Microsoft Graph as part of the request. This error often means that the access token may be missing in the HTTP authenticate request header or that the token is invalid or has expired.
We always include the token in the Authorization request header. The success rate of roughly 80% indicates that the request is formed correctly.
We strongly recommend that you use the Microsoft Authentication Library (MSAL) for access token acquisition.
We are using MSAL for .NET Core to get tokens. As I understand, the library should automatically renew expired tokens.
Are any of you able to check the logs for the specific request I've posted details about?