A change in the mod_security rules is conflicting with the Flash Uploader in the Media Manager.
The upload will fail with message "upload failed (503)"

UPDATE: The Flash uploader will not work with Flash 10. Flash 10 is a security release. The fix detailed below will not work with Flash 10.

Add this to a .htaccess file in the administrator folder to fix.

<IfModule mod_security.c>
<Files index.php>
SecFilterEngine Off
SecFilterScanPOST Off

Another option is to disable the Flash Uploader completely. This reverts the upload to the non-flash method.
Site >> Global Configuration >> System
Set Enable Flash Uploader to No.

* As of June 2009 Joomla now comes with the Flash uploader disabled.

Please note: If you have created the .htacces file in the administrator folder as descibed above and the error starts happening again, or you get error 500 when trying to access the Administrator, remove the IfModule code.

This issue may be fixed on some servers during scheduled maintenance, but still waiting to be fixed on other servers.

If you require assistance please contact me.
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