captive portal login returns sometimes wrong certificate after login

Hi friends,

we have a strange problem with our FGT50E and the captive portal.

Sometimes, when a user successfully logged in and he tries to open a web page, the Fortigate Certificate will be returned instead of the site's certificate. This will cause in a certificate error. The authenticated user have to refresh the browser 3 or more times to open the site. When we disable the authentication in the ipv4 policy (remove the group) the issue will not occour.

Not all clients in the environemnt have this issue, so we think about a possible change with any Microsoft update. We are sure, our systems does not have all the same update build.

Do you have any idea what we could check and perhaps whats going on here?

Many thanks

Thomas