We recently came across an issue with a customization that was developed for a client where invoice reports are exported to the system temp directory prior to uploading to an external system. After upgrading this client to 15.3 HF3, the custom functionality stopped working and reports weren't being generated.
After further review, we found that in 15.3 HF3, a change was implemented to improve security of the application by disallowing reports to be exported to any directory other than the FileStoragePath defined in the global XML.
Once we updated the customization code to write the reports to the FileStoragePath location, the functionality begin working again. Please keep this change in mind when upgrading clients to 15.3 HF3 and if any custom functionality like this exists in their codebase.