Clients cannot resolve many external DNS names when Windows Server 2016 DNS server is configured to use root hints.
When examining the root hints tab you discover that vast majority of the root servers listed with their IPv6 address without any IPv4 address.
When validating root hints which have IPv6 address only, it results an error.
To resolve the issue manually edit each of the roots hint and remove the IPv6 addresses, leaving only IPv4 addresses.
Validate the root hint IPv4 address from the list of root servers published by IANA.org
This issue occurs because of a recently discovered bug in the DNS service in Windows Server 2016 that is being investigated by Microsoft.