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You usually get this error when Azure encounters an error while running your web app. In order to get the detailed error messages, follow below steps to enable it.
• Login to Azure > App Services (left side menu) > Your Web App > App Service logs (search box is at the top if you can't find it), then turn on Detailed Error Messages or turn on all of the logging options, up to you.
• Now add the following in your Web Config file,
<system.web>
<customErrors mode="Off" />
<httpErrors errorMode="Detailed">
</httpErrors>
</system.web>
Finally, upload the Web Config to Azure and see if you can get detailed error messages to find the root cause.
You may check whether the deployed files are available or not using Kudu Console.
Also, make sure that your startup file (For ex: index.htm) is added to the default documents section.
Also take a look at this thread if it might help in any way - Thread
I hope this information helps. Please feel free to reach out for any questions you may have.