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.
Thanks harlan86. I feel defeated. @Sampey can you help with this?
actually i'm not sure there's a solution on bot side.
First problem is reproduce the error : only few user (you're the third) got this problem in the last six month. But the other 2 probably opens a ticket to poloniex and solve their problem.
When this error happens an html page is returned instead of an api response.
I don't know if it's possible to get the captcha image, parse it, put result inside a form and then continue. In any case it would be a lot of work with an high probability of fail.
Actually i suggest to open a ticket like the other 2 users have done.