Sorry for grave digging, but I found this thread quite on top while googling for the issue, so I thought it might help others stranding here.

Found a solution on https://forums.lawrencesystems.com/t/xcp-ng-vms-display-resolution-locked/10988/14 - albeit it refers to Windows guests, I'm pretty positive that it should work for any guest with UEFI boot configured:

• At VM start (tiano bios logo) press esc.
• Go to device manager menu
• Go to OVMF Platform configuration
• Change Preferred
• Select the resolution you want
• F10 to save
• restart the VM
• At reboot you should be fine.

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:

1. The network you choose to place the Transfer VM's virtual interface in must be reachable from your computer/XCP-ng center.
2. 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!