I signed up for the site just to reply to this as google took me here and there was no fix.
The fix is to remove all line breaks from the key mailcow provides. It should one continuous line of text - one line only. I read this in the Amazon documentation here:
https://docs.aws.amazon.com/ses/latest/dg/send-email-authentication-dkim-bring-your-own.html
“You have to delete the first and last lines (—–BEGIN PRIVATE KEY—– and —–END PRIVATE KEY—–, respectively) of the generated private key. Additionally, you have to remove the line breaks in the generated private key. The resulting value is a string of characters with no spaces or line breaks.”
Hope this helps someone else!