Ran into this issue during a fresh install of Oracle 18c. Installed ORDS into the PDB not CDB.
Tomcat 8 as application server.
Started seeing non-stop errors in the alert.log
PDB(3):WARNING: too many parse errors, count=561200 SQL hash=0xcd9b76f4 PDB(3):PARSE ERROR: ospid=29325, error=904 for statement: 2019-10-16T11:45:01.288474-04:00 PDB(3):select dbms_service_prvt.get_topology('pdb') from dual ... PDB(3):...Current username=ORDS_PUBLIC_USER PDB(3):...Application: Oracle REST Data Services Action:
Fixed it by executing "grant execute on dbms_service_prvt to ords_public_user"