@Azam Shaikh , Thank you for reaching out. Unfortunately, we do not have any samples published or available for ASP .Net WebForms. All that is available are MVC applications. You can find the list of available samples here: https://learn.microsoft.com/en-us/azure/active-directory/develop/sample-v2-code
Having said that, since ASP .NET WebForms uses the traditional an event-driven model, I believe the ASP .NET WPF sample might help.
Please find the sample here: https://github.com/azure-samples/active-directory-dotnet-desktop-msgraph-v2
Also, as you mentioned that the error comes up after the user is authenticated as "Request.IsAuthenticated" gets set to false, it would interesting to know, how the app is deciding if the user is authenticated or not. In the case of MSAL.net for ASP .NET MVC application, OWIN middleware gets implemented, which takes care of the Authentication by invoking the OpenIDConnect flow and gets the user authenticated.
But strongly recommend checking the WPF sample, which might help you or get you some understanding in the way to go ahead.
Hope this helps.
Do let us know if this helps and if there are any more queries around this, please do let us know so that we can help you further. Also, please do not forget to accept the response as Answer; if the above response helped in answering your query.