BIND 9.5.0-P2
Webmin 1.441
Mac OSX 10.5.5
If you click on the "Start Name Server" button in the BIND module, it will start a new instance of BIND each time it is clicked, instead of reloading the config and restarting BIND. This leads to multiple instances of BIND running which causes erratic behavior of the name server.
This can happen if Webmin doesn't detect that BIND is running.
Make sure that on the Module Config page, the 'Default PID file location(s)' is set correctly - it should be a path like /var/run/named.pid that contains the PID of the named process.