Cause: A privileged user killed the session and it is no longer logged in to the database.
Action: Contact the database administrator. The administrator may be attempting to perform an operation that requires users to be logged out. When the database administrator announces that the database is available, log in and resume work.
Example:
opiodr aborting process unknown ospid (4963) as a result of ORA-28
Unknown indicates here it is not a background or shadow process that is killed. ospid (4963) is the OS pid number which opiodr is aborting.
Oracle Doc ID 18233.1