A code problem would be visible in all browsers. Since this only happens in IE, it cant be a code problem.
Security Assist code is run on the server, So the browser has no effect on how that code is run.
Most likely it is the security settings in the browser. IE is notorious for having tightened security that causes sites that use logins to fail.
here is a thread from a while back that had a similar problem:
In this thread, I suggest adding the domain to the trusted sites listing in the IE Settings and setting that domain to always allow.