The happy little nag about the "do-release-upgrade" finally got me the other day. Updating the system went as expected and I worked a couple of hours fixing the obvious errors that popped up. The only thing I have no idea and can't find any documentation on is the S3 command that I set up to copy backup files from my EC2 instance over to the S3 bucket. This is the command that I have in the cron.daily script:

mysqldump -u admin -p'myPasswordHere' myDbNameHere > "$_file"
aws s3 cp "$_file" s3://my-backup-bucket/mysql/ 

This worked before the upgrade (from LTS 14.04 to LTS 16.04). Now I have the error:

upload failed: tmp/MyDbNameHere_03_16_2017.sql.gz to 
s3://my-backup-bucket/mysql/MyDbNameHere_03_16_2017.sql.gz 
An error occurred (InvalidRequest) when calling the CreateMultipartUpload operation:
Missing required header for this request: x-amz-content-sha256

I don't know how I am "missing required header" - but that seems to be the problem.

The same but different error occurs from the command line:

root@sys:~# aws s3 cp "dead.letter" s3://my-backup-bucket/
upload failed: ./dead.letter to s3://my-backup-bucket/dead.letter 
An error occurred (InvalidRequest) when calling the PutObject operation:
Missing required header for this request: x-amz-content-sha256

*edit - I did upgrade awscli to the newest version as suggested. Did not fix issue.