rndc: invalid command from xx.xx.xx.xx#1284: expired

[Michael Barrett]

Does anyone know what a message like the above means?  I can't find it
anywhere on google.  The localhost can do an rndc just fine to itself,
it's just when a remote host (who was able to do this before) tries
that it gives this error in the messages log.  The rndc client returns
the following error:

rndc: connection to remote host closed
This may indicate that the remote server is using an older version of
the command protocol, this host is not authorized to connect, or the
key is invalid.

The version on both hosts is 9.2.3, and the key has not changed.  Any
help you can give would be great.  Thanks.