What a morning...
Repeat after me, "reports is for interface, reportserver is for reportserver"...This morning I spent the best part of 30 minutes trying to determine why a SSRS report was visible through the reporting services user interface, but not when through a ReportViewer in my ASP.NET application. No matter what I tried, I kept receive a "The request failed with HTTP status 404: Not Found" error message. As a developer I contemplated every possible Report and ReportViewer property configuration before realising that I was pointing to the reporting services user interface, rather than the server itself...
There is quite a large difference between:
Hopefully, this will save someone else out there flogging the same dead horse (as the saying goes). Happy coding!