When SSO configuration is enabled on your portal, the logoff button does not actually log you off. A good workaround is to simply redirect the browser to another link. This article describes how it can be done.
- Navigate to SAP NetWeaver Administrator and go to Configuration --> Identity Management and select Configuration
- Select "Open Expert Mode" button, located on the right top side of the screen. A windows similar to the one below will appear where you need to change the following parameter: ume.logoff.redirect.url=<URL>