According to your description, when you add a DNS forwarder, unknown error occurs, but when you add the same IP address on other server, it works well.
We may perform a network traffic capture on the server that has problem and on the server works well, compare the packets when you add a DNS forwarder. In general, when you add a forwarder, the server will query for the PTR record, then send SOA query to
the remote server. If the remote server receives the packet, it will send back response-success packet. We may check if the server with problem could receive the response-success packet from the remote DNS server.
Besides, the result of validation doesnt affect forward function, when DNS server forwards query to the remote DNS server, it refer to the IP address of the remote server.
If you want to download network monitor to perform a network traffic, you may click the following link: