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:

Jul 7 08:10:46 server named[15561]: /etc/rndc.key:1: configuring key 'rndc-key': bad base64 encoding
Jul 7 08:10:46 server named[15561]: loading configuration: bad base64 encoding
Jul 7 08:10:46 server named[15561]: exiting (due to fatal error)

A simple explanation to this is that the key got modified somehow.
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.