When uploading a file, the progress area does not appear, but instead a JavaScript message window appears. The message indicates that an error has occurred and displays error details. At the end of the error message there is a suggestion to check if the RadUploadProgressHandler is registered to the application configuration file.

There are several possible reasons for this to happen:

1. The Telerik.RadUploadProgressHandler.ashx is not registered. In this case, the error message title is followed by "Source url was not found".
2. A server-side error has occurred. The error message title is followed by "Source url returned invalid content". This indicates that the server-side progress data calculation raised an unhandled exception.
   1. If measuring a custom progress, this means your progress data calculation code has raised an exception.
   2. If displaying the RadUpload progress, this indicates that the RadUpload code could have a bug.

Here are the resolutions for the different cases:

1. Handler not registered: Please, check if you have the Telerik.RadUploadProgressHandler.ashx registered in the application configuration file. Please note, that the IIS7 Integrated Mode registration should be added to the <system.webServer> section.
2. A server error has occurred:
   1. A custom progress is being monitored. Please, check the code of the progress monitoring for errors.
   2. RadUpload progress is being monitored. Please, tell us about this problem. Please, include the error message to the ticket. In Internet Explorer, this can be done by pressing CTRL+C on the error message and pasting the copied content to the body of the post. For Firefox and other browser users, please make a screenshot of the message and send it to us.

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