DotNetNuke DNN Exception Details: System.Web.HttpException: Server cannot modify cookies after HTTP headers have been sent.

We had a client with this error in their DotNetNuke Website:

Server Error in ‘/’ Application.
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Server cannot modify cookies after HTTP headers have been sent.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Web.HttpException: Server cannot modify cookies after HTTP headers have been sent.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below. 

Stack Trace:
[HttpException (0x80004005): Server cannot modify cookies after HTTP headers have been sent.]
   System.Web.HttpResponse.BeforeCookieCollectionChange() +8710212
   System.Web.HttpCookieCollection.Add(HttpCookie cookie) +24
   System.Web.Security.AnonymousIdentificationModule.OnEnter(Object source, EventArgs eventArgs) +1792
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +68
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75

For DNN the problem turned out to be in the Event Log Purge Schedule.

Under Host Settings, login and locate the history of the scheduled item. Check to see if the history shows a fail and shows this errir.

DotNetNuke.Services.Log.EventLog.PurgeLogBuffer, DOTNETNUKE
EXCEPTION: System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values. Parameter name: Index at Microsoft.VisualBasic.Collection.FastList.get_Item(Int32 Index) at Microsoft.VisualBasic.Collection.get_Item(Int32 Index) at DotNetNuke.Services.Log.EventLog.DBLoggingProvider.DBLoggingProvider.PurgeLogBuffer() at DotNetNuke.Services.Log.EventLog.PurgeLogBuffer.DoWork()

. If it does then apply this.

Turn off this item but untickign the Yes option.

Then go to the HOST / Settings Page and restart the application.

Return to the schedule and re-enable the purge log buffer with it set to perform every 3 min. Check that the schedule process finishes.

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