Hello Raghavendranath,
Are you the only one who's facing this problem or is this true for all portal users?
If it's only for your user ID, it's probably due to the fact that SSO cookie is getting stored in your temporary internet files and you can clean your temporary folder, clear browser cache and cookies, open a new session and it should work.
If this is true for all users, you can use a browser tracing tool (e.g. httpwatch professional or fiddler) to check what is happening when you click on log-off button from proxy URL.
Additionally, please share if you are you using any Web-SSO mechanism or single sign-on options?
Regards
Radhika Chhabra