If the enterprise user can connect to the database but a select * from session_roles returns no global roles, then check the following:
Check that the global role has been created in the database. To create global roles, use the following syntax:
CREATE ROLE <role_name> IDENTIFIED GLOBALLY;
Use Oracle Enterprise Manager to check that the global role is included in an enterprise role in the directory.
Use Oracle Enterprise Manager to check that the enterprise role is assigned to the user in the directory.
If these checks are fine, then see the "DOMAIN-READ-ERROR Checklist".