The Custom error pages such as 400, 401, 403, 404 and 500 that you have setup in Website Panel,

will not work for any .NET extensions ie: .aspx, asmx etc.

This is because .NET is handling the error page response and not IIS as for other file extensions.

You will need to setup your web.config to handle the needed error codes such as sample code as

below for 404 error. 

<customErrors mode="On" defaultRedirect="/ErrorPage/404.aspx"> 
<error statusCode="404" redirect="/ErrorPage/404.aspx" ></error>