This error can occur for a number of reasons:

• Firewall. Bet Angel is being prevented from contacting Betfair. Check firewall settings. Try temporarily disabling the firewall settings to see if you can now log in.
• Invalid characters in password. Do not use any of the following characters - "&%+=';£<>" or any language specific characters such as Chinese or Japanese script style characters in your password.
• Proxy. If you are behind a proxy server you may need additional settings. See here. http://www.betangel.com/knowledge-base/component/content/article/25-bet-angel/487