I've looked deeper at this problem and it seems to depend from Open Url Provider that I've installed on my DNN.

I've restyled some my websites using Open Url Rewrite on DNN 7.4.1. It's great and work fine but looking at IIS event log I can find a lot of exceptions, I think all linked to old bot indexed url and no more presents in the new website version.
Opening the URL reported in the exception I can see the right 404 page and no one issue is viewed on browser.
I have to worry about this problem? How I can fix it?
Below you can see one of this reported exceptions:

*********************************************************************
Event code: 3005
Event message: An unhandled exception has occurred.
Event time: 9/18/2015 4:50:39 AM
Event time (UTC): 9/18/2015 2:50:39 AM
Event ID: d76b3f2e1f74430491784dbbaee57c00
Event sequence: 776
Event occurrence: 8
Event detail code: 0
Application information:
    Application domain: /LM/W3SVC/12/ROOT-1-150870047247212163
    Trust level: Full
    Application Virtual Path: /
    Application Path: C:\inetpub\DNN741\
    Machine name: WEB002
Process information:
    Process ID: 4496
    Process name: w3wp.exe
    Account name: IIS APPPOOL\DNN741
Exception information:
    Exception type: HttpException
    Exception message: Server cannot append header after HTTP headers have been sent.
   at System.Web.HttpResponse.AppendHeader(String name, String value)
   at Satrabel.HttpModules.UrlRewriteModuleUtils.RewriteUrl(HttpApplication app, Uri url, String& portalAlias, PortalAliasInfo& objPortalAlias, RewriterAction& action, String applicationPath, Boolean isSecureConnection, String httpMethod, String rawUrl, String PhysicalPath)
   at Satrabel.HttpModules.UrlRewriteModuleUtils.RewriteUrl(HttpApplication app, String& portalAlias, PortalAliasInfo& objPortalAlias, RewriterAction& action)
   at Satrabel.HttpModules.UrlRewriteModule.RewriteUrl(HttpApplication app, String& portalAlias)
   at Satrabel.HttpModules.UrlRewriteModule.OnBeginRequest(Object s, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)

Request information:
    Request URL: http://www.mywebsite.com/Products/tabid/83/agentType/View/WorkID/4/SoftWork.aspx
    Request path: /Products/tabid/83/agentType/View/WorkID/4/SoftWork.aspx
    User host address: 60.168.59.196
    User: 
    Is authenticated: False
    Authentication Type: 
    Thread account name: IIS APPPOOL\DNN741
 
Thread information:
    Thread ID: 3
    Thread account name: IIS APPPOOL\DNN741
    Is impersonating: False
    Stack trace:    at System.Web.HttpResponse.AppendHeader(String name, String value)
   at Satrabel.HttpModules.UrlRewriteModuleUtils.RewriteUrl(HttpApplication app, Uri url, String& portalAlias, PortalAliasInfo& objPortalAlias, RewriterAction& action, String applicationPath, Boolean isSecureConnection, String httpMethod, String rawUrl, String PhysicalPath)
   at Satrabel.HttpModules.UrlRewriteModuleUtils.RewriteUrl(HttpApplication app, String& portalAlias, PortalAliasInfo& objPortalAlias, RewriterAction& action)
   at Satrabel.HttpModules.UrlRewriteModule.RewriteUrl(HttpApplication app, String& portalAlias)
   at Satrabel.HttpModules.UrlRewriteModule.OnBeginRequest(Object s, EventArgs e)
   at System.Web.HttpApplication.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
Custom event details:
*********************************************************************

Yes sure. I posted also here because unfortunately I opened the thread at first here before to find the exception origin.
The thread goes on here : https://openurlrewriter.codeplex.com/...
Thanks