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Check the current roles assigned to the user. Log in as an admin user and select the 'Users' option in the Analyze Directory's navigation panel then select the user who is reporting the login issue. The roles shown in the Properties panel must include the 'User' role else the user will not be able to login even if other roles are granted due to their group membership. If this is not present, select the 'Edit' button and add the User role to that user profile.
Also, check the setting for the 'Default roles for new accounts' property in 'Settings' -> 'User Accounts'. If this property is set to include the 'User' role then any newly created users will be able to log into the system (provided they are authenticated) and their effective access control permissions will be determined by their group membership.