I don't use Windows. I investigated a bit the JS code and there's an issue with the way the headers are generated for the AJAX calls that fetches the captcha picture. Changing at line 18 of captcha.js the following "Last-Modified": new Date(0), "If-Modified-Since": new Date(0) to "Last-Modified": (new Date(0)).toISOString(), "If-Modified-Since": (new Date(0)).toISOString() fixes it. Otherwise you get a "TypeError: Cannot convert string to ByteString because the character at index 50 has value 8217 which is greater than 255." probably because without the toISOString method date contains UTF-8 characters instead of ASCII ones (which are required for HTTP headers).