Oracle Solution for Error ora-12521

ORA-12521: TNS:listener does not currently know of instance requested in connect descriptor

What triggered the Error:

The listener received a request to establish a connection to a database or other service. The connect descriptor received by the listener specified in addition to the service name an instance name for an instance (usually a database instance) that either has not yet dynamically registered with the listener or has not been statically configured for the listener. This may be a temporary condition such as after the listener has started, but before the database instance has registered with the listener.

What should we do to fix it:

*- Wait a moment and try to connect a second time. *- Check which instances are currently known by the listener by executing: lsnrctl services *- Check that the INSTANCE_NAME parameter in the connect descriptor specifies an instance name known by the listener. *- Check for an event in the listener.log file.

If this was not sufficient go over to: Oracle Official Support Knowledge Center
Or maybe those geeks at: StackOverflow Board