The utility db-control online-backup or backup on Red Hat Satellite 5.6 does not work when using an external database. Why?

Issue

• The utility db-control online-backup or backup on Red Hat Satellite 5.6 does not work when using an external database. Why?
• Trying to get a database backup the db-control online-backup throws the error below:

# db-control online-backup /tmp/online-backup.sql
Backing up to file /tmp/online-backup.sql.
pg_dump: [archiver (db)] connection to database "rhnschema" failed: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/tmp/.s.PGSQL.5432"?
pg_dump: *** aborted because of error
Backup complete.