It says on the S3 docs about SSE-C
If you lose the encryption key, any GET request for an object without its encryption key fails, and you lose the object.
Which kind of implies it isn't possible to read the object without its encryption key
I'm reasonably sure that this error
InvalidArgument: Requests specifying Server Side Encryption with Customer provided keys must provide a valid encryption algorithm.
Comes from minio itself as I found that message it its source code in the cmd/api-errors.go file.
So alas I don't think it is possible to read the encrypted objects.
It might be worth asking on the minio forum or as an issue if it is possible to read the encrypted objects for backup purposes...
So I think you'll need to get the key from the customer if you want to back up using the S3 API.