After upgrading a CQ5.2.1 authoring instance to 5.3, a basic login prompt is displayed rather than CQ's default login page.
The path to the default login page has changed from CQ5.2.x to CQ5.3:
The SSO configuration of the SSO Authentication Handler stores the path to the default login-page in case no valid credentials are found in the request. After the upgrade to CQ5.3, the default login-page still points to the old location.
Edit the SSO Authentication Handler configuration via the Felix Console and reconfigure the path to the default login-page: