Error on download of file from S3 storage

On 12.0.4, I added an S3 bucket as a folder. I uploaded a file, and the file appears fine on the S3 bucket. When I click the file in the web interface however, I get an error (no error code):

Service Unavailable

This is a plain white error page with the client IP, and a request ID after the above text. Ideas?

No answers at all, but after upgrading to 13RC1, the error disappeared.