It looks like you haven't allowed authoritative requests from off your network.
I don't use MyDNS or whatever ISPConfig uses for DNS, but if it's bind, have a look in your config at the options statement and check your external facing interface is in the listen-on list, and that allow-query (if it exists) lists who should be able to access it. Usually, you would want "any;" for a publically accessable authoritative name server.
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