Any chance you have Webmin installed on that server? I find it to be an essential complement to ISPConfig.
If so, you might use Webmin to check the records for problems. Within Webmin, go to Servers -> BIND DNS Server -> Check BIND Config.
Networking and DNS are not my specialty, so I'm pretty much at the limit of my usefulness here. However, I think it's safe to say that this is a DNS problem at this point.
Falko?
