Server Cannot Append Header After HTTP headers have been sent Exception at @Html.AntiForgery

When Response.Redirect(anyUrl) the status code is set to 302, and the header will added to the response :

HTTP 1.0 302 Object Moved 
Location: http://anyurl.com

And when ViewResult is executed and razor render the view the Html.AntiForgeryToken() will called, so the helper tries to add the header X-Frame-Options and some cookies to the response, it is the cause of the exception.

But don't worry you can suppress the addition of X-Frame-Options header, just put this AntiForgeryConfig.SuppressXFrameOptionsHeader = true; in the Application_start.

But I suggest you to change this:

Response.Redirect(returnUrl);

to

return Redirect(returnUrl);

Note

Since .NET code was opened you can see how the AntiForgeryToken works, see here AntiForgeryWorker.

I was getting same error with Response.Redirect(returnUrl). After changing to Response.Redirect(returnUrl, false) fixed the issue.