It's very hard to troubleshoot those or to find solutions for them, even if you have incredible internet search skills. That said, this solution may not work for you, but hopefully will give you a quick test to see if you can move on to another suggestion.
I was puzzled while trying to solve a problem we had in our web application where exporting via ActiveReports 8 WebViewer to PDF or DOC worked fine, but XLS for some reason returned a 404 error.
The problem turned out to be with mine that we missed laying down a DLL required by the Excel exporter (GrapeCity.ActiveReports.Export.Excel.v8.dll) called DocumentFormat.OpenXml.dll, which didn't seem to be referenced by the other Export DLLs.
As soon as I put this in place, it started working again.
This file could be easily missed as it is not named like the other export related DLLs:
Make sure to include that and you'll have one less reason to see a 404 error when you try exporting to Excel from ActiveReports!
On a side note, the following are all of the referenced DLLs (Microsoft DLLs excluded) for GrapeCity.ActiveReports.Export.Excel.dll in case you need to check for others also missing:
Best of luck!