app_offline.htm file does not work

Create a web.config file with following content

<?xml version="1.0"?>
<configuration>
    <system.webServer>
        <modules runAllManagedModulesForAllRequests="true" />
    </system.webServer>
</configuration>

Hope that helps.

I had the same issue, and although I couldn't solve it, I found a reasonable workaround. I added the same file, but named "appoffline.htm" to the root directory and leave it there permanently.

When I need to take application offline, I use the IIS HTTP Redirection setting for the website to redirect all incoming requests to appoffline.htm (make sure to tick "Redirect all requests to exact destination").

I was also struggling a little bit with this issue.

Along with the fore-mentioned criteria in the other answers. It turns out that the file type MUST be specified specifically .htm NOT .html.

I recently had the same issue with the app_offline file and the real problem I had was that windows was set to hide known file extensions. So when the file app_offline.htm was created I thought that the name was correct, but windows was hiding the extension .txt.