Cause: The INIT.ORA parameter "thread" is zero, its default value. There are no threads which have been publicly enabled, and not mounted. See the solution below.
Error solution: Shutdown the instance, change the INIT.ORA parameter to a thread which is privately enabled and not mounted. If the database is open in another instance, then a thread may be publicly enabled.
Database version: Oracle 11g Release 2 (11.2)