Customer reports they are not using SSO / Multi-Provider SSO yet when a user try to access ServiceNow instance in browser, user is redirected to external_logout_complete.do page stating below message on screen:

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Logout Succeeded

Logout Successful

You have successfully logged out.

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Applicable to all releases

SSO / Multi-Provider SSO is one of the external authentication methods available and not the only one. Therefore, in this case when Multi-Provider SSO is not active, it confirms some other external authentication is active which is broken or misconfigured. System property glide.authenticate.external controls enabling / disabling external authentication.

For a quick relief, set system property glide.authenticate.external value as false and check what other external authentication is customer using and you can fix it according.