Credential must have exactly 5 slash-delimited elements

I am getting this error randomly when using the sendEmail API call. It works most of the time, but there's 1 specific email type that goes out in the morning that gives this error every day. Then if I send the same email later in the day with no change to anything, it works fine. If my credentials were incorrectly set I would get this error every time, what would cause it only to happen some of the time and then work when I try it again? I'm using this API call exactly the same for every kind of email I send.