Error - Service is currently unavailable, try again later. manage user screen in VBCS DCS App 25D

Summary:
Hi,
we have a customer self service portal in oracle fusion service cloud. on the manage user tab is visible only for the contacts with the "Customer Self-Service Account Administrator" role. we here getting the below error when an contact with the Administrator role clicks on "manage users" tab.

Error MSG : "Service is currently unavailable, try again later."

The role "Customer Self-Service Account Administrator" (ORA_SVC_CUSTOMER_SELF_SERVICE_ACCOUNT_ADMINISTRATOR_ABSTRACT) having below privileges :

SVC_APPROVE_SELF_SERVICE_REGISTRATION_USING_REST_PRIV
SVC_CREATE_SERVICE_ROLE_USING_REST_PRIV
SVC_DELETE_SERVICE_ROLE_USING_REST_PRIV
SVC_UPDATE_SELF_SERVICE_ROLE_USING_REST_PRIV