asp.net form authenciation cookie does not work in ie8

[tandor]

In our asp.net application,we use the form authenciation.And we use the iframe for layout ,but all the link of the src is inside our domain.Sometimes if user login using ie8,he will always redirect to the login page,then I use the developer tool,I found that no cookie is set in the browser.However,in some other machine user can login with ie8.It works well in firefox and ie7.And the name of our server is \[code\]sbl21\[/code\],no un-allowed characters.This is the form config:\[code\]<forms name="LoginCookieName" loginUrl="login.aspx" timeout="40" />\[/code\]If we add the \[code\]cookieless="UseUri"\[/code\], then user can login,but if he click any links in the page,a alert box will dispaly which show user is not logined.What is the problem?