When trying to restart named process after making modifications may end up in a corrupt rndc.key key and the error will show like this:
Sep 12 03:30:54 server named: loading configuration: bad base64 encoding
Sep 12 03:30:54 server named: exiting (due to fatal error)
A simple explanation to this is that the key got modified somehow might me some bug.
What to do about this? Well it is simple just check the /etc/rndc.conf file and copy the key from there(you will see the key in the first lines of the file) and replace the key that it is in /etc/rndc.key file and restart named process.
# service named restart
Stopping named: [ OK ]
Starting named: [ OK ]