Sorry for grave digging here, but since this answer ranks pretty high in the Google results, I thought I might add a possible solution:
During the export wizard in XCP-ng Center, you're being asked for the network configuration of the Transfer VM. As for my observations, two things are pretty vital here:
- The network you choose to place the Transfer VM's virtual interface in must be reachable from your computer/XCP-ng center.
- Enter a static IP configuration, DHCP has never worked for me, even though a DHCP server is running in the selected network.
Hope this helps some!