DMZ backup fails

[ianbutton1]

Veeam is working pretty well for our internal network VMs (on VMware and Hyper-V). Now I want to get it to backup VMs in the DMZ (Hyper-V hosts), but it isn’t working.
After creating snapshots OK it fails, with the message “Processing guestVMname Error: read: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond”, and the job log says “

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<11> Error    Processing[i] guestVMname[/i]
<11> Error    Failed to connect to server '[i]GatewayServerToStoreOnce[/i]', addrs '[IP:[i]xx.xx.xx.xx[/i], port:2501];[IP:[i]GatewayServerToStoreOnce[/i], port:2501]' (Veeam.Backup.Common.CAgentException)
. . . . .
. . . . .
<11> Error    read: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond (Veeam.Backup.Common.CCppComponentException)

We have opened our DMZ firewall ports 2500-2550 for traffic (bothways) between the VBR server & StoreOnce Gateway server on the internal network, and the guest VM & Hyper-V proxy server in the DMZ (the job is configured for on-host proxy). The job log implies that it is the Gateway server that something (DMZ guest? Proxy? VBR server?) can’t connect to. But I don't know which!
All the guidance suggests checking that port 2500 (for example) is open in the DMZ firewall, by telnetting across, from VBR server to VM & back, etc. But when I try that, it always says “Connection failed”. My network colleagues see a TCP request going through the firewall, and a TCP reset being returned, to fail the connection! Netstat, resmon etc do not show any listener on TCP port 2500 on any of the machines, UNLESS (as suggested by Veeam Support) I run VeeamAgent.exe on the DMZ guest, where it then displays “Dispatch Port:2500” but nothing else, and if I telnet from the VBR server to that, the command windows just stays blank (no failure error).
We have no local (Windows) firewall active on any of the machines involved, and MS Enhanced Security Configuration is off.
If Veeam is working fine inside the network, and we have opened the necessary DMZ firewall ports etc, what on earth can be wrong? What can be stopping TCP traffic so that both telnet and Veeam fail on port 2500+? How can I troubleshoot this? [Veeam case is 03310530]
TIA

[ianbutton1]

Sorted now - it was a DNS problem: Hyper-V host had entries in its hosts file, for VBR server and Gateway server, but these were FQDN entries, not the shortname that Veeam was using (& showed in the log). Adding the shortname in each case solved the problem.