Snapshot does not support the AWS4-HMAC-SHA256 authentication method.
AWS4-HMAC-SHA256, also known as Signature Version 4, ("V4") is one of two authentication schemes supported by S3.
All regions support V4, but US-Standard, and many -- but not all -- other regions, also support the other, older scheme, Signature Version 2 ("V2").
According to http://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-authenticating-requests.html ... new S3 regions deployed after January, 2014 will only support V4.
Since Frankfurt was introduced late in 2014, it does not support V2, which is why I can't use Snapshot Pro for my Frankfurt bucket... that contains ALL my clients backups (I previously used BackupBuddy, which supports V4)