THANK YOU! This solved it for me. /root/.aws/credentials looked like this:
[MYACCESSKEY]
aws_access_key_id = MYACCESSKEY
aws_secret_access_key = MYACCESSKEY
[default]
aws_access_key_id = MYACCESSKEY
aws_secret_access_key = MYSECRETACCESSKEY
I copied the real secret access key into the first section and backing up to S3 now works. I don’t recall getting the secret access key wrong when I set it up (though it was a while ago) but it was certainly correct in Virtualmin.
I find it strange that I was getting an error message telling me to use AWS4-HMAC-SHA256 as opposed to an error that my secret key was wrong but at least it’s working.