If you have access to a command line in your server please try executing the command manually to see what it prints. The commands would be:
cd (opengoo path)
mysqldump --host=(host) --user=(user) --password=(password) (database) > tmp/db.sql
The values for (host), (user), (password) and (database) are the ones in your config/config.php. Replace (opengoo path) for the path to your OpenGoo installation.