RMAN cannot browse database, ORA-01034 error (4046016)

When trying to expand or browse the RMAN plugin in the backup window, a popup window appears with the errors:

ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist

On Solaris there is a further message:
SVR4 Error: 2: No such file or directory

1. Makes sure Oracle is running
2. Check that the ORACLE_SID is specified correctly in the database configuration in the plugin.
3. Check that the Oracle database is up and running and that you can log in using Sqlplus
4. Check that the ORACLE_HOME environment variable in the database configuration of the plugin does NOT have a trailing slash (This does not matter on Windows OS)
5. Check that the user ID and password are correct.