Thanks for the reply.
I did a little more digging and found that it had something to do with Cisco Umbrella Security and a reference to OpenDNS. I forgot I had set OpenDNS familyshield as the upstream DNS, I switched to OpenDNS and still the same problem. But when I set as Google upstream, discord is back to normal.
So I think the problem was that OpenDNS has implemented some discord blocking.
So yeah, your query about the upstream server was right.