Runtime Error while trying to pull the custom error page in IIS 7 in windows server 2008 R2

I am using the below code to display the 404 custom error page.

<customErrors mode="On">
      <error statusCode="403" redirect="../403.htm" />
      <error statusCode="404" redirect="Unsecure/404.html" />

i am trying to use the below url's after i hosted is my original url but when i use i am able to see the 404 page but when i am trying to use the url is added at the end) i am getting the below error

Server Error in '/' Application.
Runtime Error
Description: An exception occurred while processing your request. Additionally, another exception occurred while executing the custom error page for the first exception. The request has been terminated. 

I have hosted this in window 2008 server R2.