Originally Posted by _ronnie_
When I try to echo the contents of $mod->system->server_conf["server_bind_zonefile_dir"], I get a null result. The server_bind_zonefile_dir value in the isp_server table is empty. Should that be the case?
No. In DEBIAN it is /etc/bind
Check where your zonefiles are and put this directory in the isp_server table (without trailing slash).