If using the Auth0 Universal Login Page, try turning off customization and see if authentication works. If login works without your customizations, review your Page customization code.
If the logged in user cannot access another application with Single Sign-on
Is the user trying to login to the second application from the same browser as their initial login?
Go to your Tenant Settings > Advanced Settings and check the Log In Session Management settings. Was the second login attempt within the timeout periods?
Check the value passed as the prompt parameter in /authorize call.
Is the connection used to log in to the first application enabled for the second application?
Did the second application receive all the necessary user profile information?
If using a mobile device, you’ll need to use a browser-based flow. For more information, see Best Practices.
Did you add your logout redirect URLs to the allow list? If you are using a redirect URL in a logout call it must be registered in either the tenant or application settings.
If you need federated logout, did you append the ?federated parameter to the logout call?
Make sure that the logout redirect URL is different from the login callback URL.
Make the logout redirect URL an anonymous page (not protected by login) so that redirects to the logout redirect URL do not immediately trigger a login, which may confuse users.