Thanks for reporting!
The issue seems to be the same as what was reported at: File-downloader failed to download keystore, cannot find source url
In short we had a faulty deploy which made signing files uploaded after the deploy not accessible in the builds. Files uploaded before that deploy still worked, just not the ones uploaded after that deploy went into production.
The issue is fixed now (as reported at File-downloader failed to download keystore, cannot find source url), a simple rebuild should work.
Sorry for the inconvenience and let us know if you’d have any questions!