Yup - same for me too, existing Amazon account in captcha loop - can't add new Amazon accounts either. Wish we could get a handle on what appears to be a repeating problem with no solution. Don't ever remember seeing anything posted anywhere - that gets me out of this loop. Seems to just go away in time - but not fast enough for me.
Update 3/10/17: As usual, problem magically resolves itself
(at least for me)