I got this error on DGMGRL command line when I was configuring 11g Dataguard and I issued
DGMGRL> show configuration;
This error was coming as my standby database was started using pfile,
I shutdown the standby database and restarted it using spfile and the problem was resolved.
Another time I got this error was when I changed the IP address of the machine after dataguard had been configured. I deleted the tnsnames.ora and listener.ora entries and recreated them. Then it started working.