Is the server listening for 6pn connections on IPv6 (:: or fly-local-6pn instead of 0.0.0.0)? See: Fly io Newbie: Making internal requests between apps - #3 by ignoramous Consequently, is the client capable of making IPv6 connections?
If so, and if you are deploying Apps v1 (nomad), then may be wait it out a bit; ref: Can anyone from Fly confirm if our support email is actually working?