Logout is not working in portal

Hello,

I have a custom masthead in my CE73 Portal containing a logout linke where user can click and it gets redirected to logon screen.

(note: we have windows kerberos authentication in place for portal)

Somehow since last week, this logout doesn't work anymore.

When user clicks on logout, portal just re-loads and comes back to same screen with same user.

I checked HTT trace using Fiddler2 tool (attached).

Any ideas/hint on this topic would be greatly appreacited.

Thanks & Regards,

Mohiyan