FIM Portal Internal Error

I've configured my portal to have a set of HR users. HR users can access the portal, create users and modify certain attributes of existing Contractors and Staff.

To do this I created some MPRs and search scopes, I login as an HR user, click Users and can successfully create a new user. However, if I search for existing users using the default All Users search scope, or using my All Conteactors and All Staff search scopes, the portal returns:

An internal error occurred and your request cannot be processed. Please contact your system
administrator.

Usual objectSid, Domain, AccountName are in place. Am I missing something simple?

Thanks