Custom Error Page not showing - asp.net

Question

For some reason, when I get an ASP.NET runtime error, it's not loading my custom error page
<customErrors mode="On" defaultRedirect="app_offline.htm" redirectMode="ResponseRewrite">
<error statusCode="404" redirect="app_offline.htm"/>
<error statusCode="500" redirect="app_offline.htm"/>
</customErrors>
That's in my web.config.
I'm still getting this though and it's not loading my error .htm page:
Server Error in '/' Application.
Runtime Error
Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed remotely (for security reasons). It could, however, be viewed by browsers running on the local server machine.
Details: To enable the details of this specific error message to be viewable on remote machines, please create a <customErrors> tag within a "web.config" configuration file located in the root directory of the current web application. This <customErrors> tag should then have its "mode" attribute set to "Off".
<!-- Web.Config Configuration File -->
<configuration>
<system.web>
<customErrors mode="Off"/>
</system.web>
</configuration>
Notes: The current error page you are seeing can be replaced by a custom error page by modifying the "defaultRedirect" attribute of the application's <customErrors> configuration tag to point to a custom error page URL.
<!-- Web.Config Configuration File -->
<configuration>
<system.web>
<customErrors mode="RemoteOnly" defaultRedirect="mycustompage.htm"/>
</system.web>
</configuration>

Answer 1

I'm pretty sure that app_offline.htm is a reserved filename in ASP.NET that will always be served if present on the server.
Try renaming it to error.htm and updating your <customErrors /> block to match.

Answer 2

I suspect what's happening is that ASP.NET can't find your custom error page. The path to your error page file needs to be relative or absolute. So either:
<customErrors mode="On" defaultRedirect="~/app_offline.htm" redirectMode="ResponseRewrite">
<error statusCode="404" redirect="~/app_offline.htm"/>
<error statusCode="500" redirect="~/app_offline.htm"/>
</customErrors>
Or:
<customErrors mode="On" defaultRedirect="http://mysite.com/app_offline.htm" redirectMode="ResponseRewrite">
<error statusCode="404" redirect="http://mysite.com/app_offline.htm"/>
<error statusCode="500" redirect="http://mysite.com/app_offline.htm"/>
</customErrors>
Should solve your problem.

Answer 3

Do you get details of the actual error if you set mode="Off"? If you still get the same default error page, you might find the solution in this post:
CustomErrors mode="Off"

Answer 4

Problem was my site was running under .NET 3.5 when it's a 4.0 app

Answer 5

I also had the same issue. Main problem is related to visibility of your error pages to various users. It is controlled by the tag
Mode
as shown below
I think you have used RemoteOnly . In order to show the custom error page while running in your local machine we have to make the mode value as On. For me it is worked afterwords.
Also we can give error pages for various types of error codes. For example in below figure you can see - I have given redirecting page as CustomErrorAspxPage.aspx for file not found exception type for the status code 404.
At last in order to test this I created exceptions 1)for default exception 2)for 404 file not exists exception as shown in below figure.