Server cannot append header after HTTP headers have been sent.

hi.
i have a web forms app (deployed to azure) that was created using vs 2013 update 2. i have also injected elmah into the app. now it seems something is wrong with the way authentification works: _sometimes_ i get the following exception notification from elmah:
System.Web.HttpException: Server cannot append header after HTTP headers have been sent
System.Web.HttpException (0x80004005): Server cannot append header after HTTP headers have been sent.    at System.Web.HttpHeaderCollection.SetHeader(String name, String value, Boolean replace)    at System.Web.HttpHeaderCollection.Set(String name, String value)    at Microsoft.Owin.Host.SystemWeb.CallHeaders.AspNetResponseHeaders.Set(String key, String[] values)    at Microsoft.Owin.Host.SystemWeb.CallHeaders.AspNetRespon ...