polo added additional protection against ddos which require captcha checking before logic. it affect all bots as well.
you could use proxy or vpn for avoid login problem or ask Sampey to implement captcha checking on polo with cat.
A captcha to a API request sounds extremely stupid, doesn't it? Must be an error at Poloniex. I suggest to file a support ticket, although, they hardly answer to any technical issue (at least not to me: My technical ticket is in the open status since a month).