原因分析：
服务器开启了Https时，cookie的Secure属性应设为true；
解决办法：
1.服务器配置Https SSL方式，参考：https://support.microsoft.com/kb/324069/zh-cn
2.修改web.config，添加:
<system.web>
<httpCookieshttpOnlyCookies="true" requireSSL="true" />
<system.web>
See: http://msdn.microsoft.com/en-us/library/ms228262(v=vs.100).aspx
3.修改后台写Cookies时的设置 cookie.Secure = true：
HttpResponse response= HttpContext.Current.Response;
var cookie = new HttpCookie(key, value);
cookie.HttpOnly = true;
cookie.Path = "/";
cookie.Expires = DateTime.Now.AddHours(1);
cookie.Secure = true;
response.AppendCookie(cookie);
参考网站：http://stackoverflow.com/questions/5978667/how-to-secure-the-asp-net-sessionid-cookie