Based on the attached screenshots it seems the report throws an exception during processing. The REST service works fine, as shown in Fiddler, but the processing of the report fails.
Error 404 can be returned when a web content has been requested in the report. This could be a WebServiceDataSource, CSVDataSource or a PictureBox, pointing to an invalid URL. Please check if the report contains any of these items and make sure they are able to retrieve its contents correctly. The recommended way of obtaining more information about the exceptions is to attach a trace listener to the application configuration file, as explained here: How to: Create and Initialize Trace Listeners
Hope this helps.
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