In case anyone else is looking for an answer to this in the future, the message:
(13) The merchant login ID or password is invalid or the account is inactive.
evidently appears for more reasons than supplying the wrong login id or connecting to the wrong url at least as far as DPM is concerned.
In my instance, the refresh on my link "https://www.my.com/CCPayment2.vbhtml" contained an invalid URL. Once I discovered and fixed this, everything worked fine.