Start by looking into the WFDS tables to see if the data is properly set. Check views wf_user_roles and wf_user_role_assignments_v to see that the ad hoc role has been assigned properly to the two users. If they are here then the issue would be with the Notification system. What apps version are we talking about?
I am facing same error. I am using standard workflow API to create and assign a role functionality for sending notification to Multiple users.
Error Message = ORA-06500: PL/SQL: storage error
ORA-04030: out of process memory when trying to allocate 16328 bytes (koh-kghu call ,pmucalm coll)
Error Stack =
I would appreciate that if know what could be the issue.