Collect Engine may fail with
Unable to Retrieve Database Properties from Manager: <name of Domain Manager>. COAPI Returned:25
After a restart of the engine, it may work for a while but fail again later on.
You will find some ORA-12516 msgs in the logs.
This msg appears:
ORA-12516: TNS:listener could not find available handler with matching protocol stack
How to increase the stack handler value
You need to increase the stack handler value for Oracle.