These errors are seen in the velero pod logs after customer has upgraded their OADP operator from 1.3 to 1.4,

message: 'BackupStorageLocation "together-forever-1" is unavailable: rpc error: code = Unknown desc = operation error S3: ListObjectsV2, https response error StatusCode: 403, RequestID: , HostID: , api error SignatureDoesNotMatch: Access denied.' phase: Unavailable

We had them double check their secret since according to [1] it could be a conflict in their AWS secret access key and signing method. 

Using the same set of keys, the error went away when they downgraded back to 1.3.

[1]https://docs.aws.amazon.com/AmazonS3/latest/API/ErrorResponses.html#ErrorCodeList