PageHandlerFactory-Integrated-4.0 Server Error

I am new to web.config, I am taking over the code from another developer. I have this Internal Server when I try to access one of the pages.

HTTP ERROR 500.19 - Internal Server Error

Module: CustomErrorModule Notification: SendResponse Handler: PageHandlerFactory-Integrated-4.0 Error Code: 0x80070021 Config Error: Lock Violation

Logon User: Anonymous

Here is more info on the error:

System.Web.HttpParseException" message="The base class includes the field 'Iframe', but its type (System.Web.UI.HtmlControls.HtmlGenericControl) is not compatible with the type of control (System.Web.UI.HtmlControls.HtmlIframe)."

There is no iFrame in my aspx page, and there is no iFrame or HtmlFrame in the designer page. I don't know what to do.

Other pages of the website are using the same design and are accessible without error.

Try this:

Edit the file C:\Windows\System32\inetsrv\config\applicationHost

In the <configSections> make the below changes

<section name="handlers" overrideModeDefault="Deny" /> change this to "Allow"

<section name="modules" allowDefinition="MachineToApplication" 
  overrideModeDefault="Deny" /> change this to "Allow"