The issue has been resolved. Internal connectivity issues between two servers on one particular cluster had caused it. Recently uploaded files were not stored and served correctly, so returned 415 errors for those new files. Any recently uploaded images with 0 bytes should be reuploaded. Only 6 Sirv accounts were impacted and the account owners have been informed. Fewer than 0.01% of all requests to those accounts were impacted and no other accounts were impacted.