API down status appears on this reasons:
1) Exchange is really down and or there DDOS attack.
2) There is to match requests from your IP to exchange.
3) There is DNS or provider problem.

Solutions:
1) No solution, you mast wait until server begin works again.
2) This problem happens if your provider give to clients single IP address and many of them use api with same exchange. Currently capcha is not supported, but I'll add this soon.
For now there is two solutions:

• Make requests interval larger. Go to %localappdata%\QtBitcoinTrader and edit ini file. And set value HttpRequestsInterval to 1000 for example. Or larger if required.
• Only for advanced users. Go to exchange web site and enter capcha. Now you mast get UserAgent and Cookies from your browser and set "Network" section values in ini file: UserAgent=".." and Cookies=".."