Web.config registration missing!
 The Telerik dialogs require a HttpHandler registration in the web.config file. Please, use the control's Smart Tag to add the handler automatically, or see the help for more information: Controls > RadEditor > Dialogs > Introduction

<system.web>
 <httpHandlers>
      <add path="Telerik.Web.UI.WebResource.axd" verb="*" type="Telerik.Web.UI.WebResource, Telerik.Web.UI" />
      <add path="Telerik.Web.UI.SpellCheckHandler.axd" type="Telerik.Web.UI.SpellCheckHandler" verb="*" validate="false" />
      <add path="Telerik.Web.UI.DialogHandler.aspx" verb="*" type="Telerik.Web.UI.DialogHandler, Telerik.Web.UI" validate="false" />
    </httpHandlers>
    <httpModules>
      <add name="RadUploadModule" type="Telerik.Web.UI.RadUploadHttpModule, Telerik.Web.UI" />
    </httpModules>
  </system.web>
  <system.webServer>
    <validation validateIntegratedModeConfiguration="false" />
    <handlers>
      <add name="Telerik.Web.UI.DialogHandler.aspx" path="Telerik.Web.UI.DialogHandler.aspx" verb="*" type="Telerik.Web.UI.DialogHandler, Telerik.Web.UI" />
      <add name="Telerik_Web_UI_SpellCheckHandler_axd" verb="*" preCondition="integratedMode" path="Telerik.Web.UI.SpellCheckHandler.axd" type="Telerik.Web.UI.SpellCheckHandler" />
    </handlers>
    <modules runAllManagedModulesForAllRequests="true">
      <add name="RadUploadModule" type="Telerik.Web.UI.RadUploadHttpModule, Telerik.Web.UI" preCondition="managedHandler" />
    </modules>
  </system.webServer>
</configuration>

routes.Ignore("{resource}.axd/{*pathInfo}");
routes.Ignore("Telerik.Web.UI.DialogHandler.aspx");

HTTP Error 500.21 - Internal Server Error
Handler "Telerik.Web.UI.DialogHandler.aspx" has a bad module "ManagedPipelineHandler" in its module list

<httpHandlers>
      <add verb="*" path="Telerik.RadUploadProgressHandler.ashx" type="Telerik.Web.UI.Upload.RadUploadProgressHandler, Telerik.Web.UI"/>
      <add verb="*" path="Telerik.Web.UI.DialogHandler.aspx" type="Telerik.Web.UI.DialogHandler, Telerik.Web.UI, Culture=neutral, PublicKeyToken=121fae78165ba3d4"></add>
      <add verb="*" path="Telerik.Web.UI.SpellCheckHandler.axd" type="Telerik.Web.UI.SpellCheckHandler, Telerik.Web.UI, Culture=neutral, PublicKeyToken=121fae78165ba3d4"></add>
      <add verb="*" path="ChartImage.axd" type="Telerik.Web.UI.ChartHttpHandler, Telerik.Web.UI, Culture=neutral, PublicKeyToken=121fae78165ba3d4" validate="false"/>
      <add path="Telerik.Web.UI.WebResource.axd" verb="*" type="Telerik.Web.UI.WebResource, Telerik.Web.UI" validate="false"/>
    </httpHandlers>
    <httpModules>
      <add name="RadCompression" type="Telerik.Web.UI.RadCompression"/>
      <add name="RadUploadModule" type="Telerik.Web.UI.RadUploadHttpModule, Telerik.Web.UI"/>
    </httpModules>

<modules>
      <add name="RadCompression" type="Telerik.Web.UI.RadCompression"/>
      <add name="RadUploadModule" type="Telerik.Web.UI.RadUploadHttpModule, Telerik.Web.UI" preCondition="managedHandler"/>
    </modules>
    <handlers>
      <add name="ChartImage.axd_*" path="ChartImage.axd" verb="*" type="Telerik.Web.UI.ChartHttpHandler, Telerik.Web.UI, Culture=neutral, PublicKeyToken=121fae78165ba3d4" preCondition="integratedMode"/>
      <add name="Telerik.Web.UI.SpellCheckHandler.axd_*" path="Telerik.Web.UI.SpellCheckHandler.axd" verb="*" type="Telerik.Web.UI.SpellCheckHandler, Telerik.Web.UI, Culture=neutral, PublicKeyToken=121fae78165ba3d4" preCondition="integratedMode"/>
      <add name="Telerik.Web.UI.DialogHandler.aspx_*" path="Telerik.Web.UI.DialogHandler.aspx" verb="*" type="Telerik.Web.UI.DialogHandler, Telerik.Web.UI, Culture=neutral, PublicKeyToken=121fae78165ba3d4" preCondition="integratedMode"/>
      <add name="Telerik.RadUploadProgressHandler.ashx_*" path="Telerik.RadUploadProgressHandler.ashx" verb="*" type="Telerik.Web.UI.Upload.RadUploadProgressHandler, Telerik.Web.UI" preCondition="integratedMode"/>
      <add name="Telerik.Web.UI.WebResource.axd_*" path="Telerik.Web.UI.WebResource.axd" verb="*" type="Telerik.Web.UI.WebResource, Telerik.Web.UI" preCondition="integratedMode"/>
    </handlers>