IAM keys - same user, new key, can't authenticate!

I will start my saying I know I should be using IAM roles and we are converting a LOT, some will take a little longer so I want to rotate them out due to them getting old. So this key simply has access to AmazonSesSendingAccess and things are working as expected. I have a simple python script that sends mail to confirm.

From that IAM user, I select [security credentials], I see the existing, select Create Access Key and with the menu simply select the using on an EC2 and checking the box saying I know I am creating a key. I download the CSV as well use the copy icon and paste replacing the old one.

Now when I test I am getting the following; Error: (535, b'Authentication Credentials Invalid')

I uncomment the old and things work as expected. I see nothing else as its the same user (so permissions are the same), and I am 100% stuck. Any help or suggestions are appreciated.