I was having this problem too. I used OVFTool 4.3.0 to export a VM to ova. I am not sure if it's just coincidence or not, but the first ova imported fine using sources.
However, I exported two more VMs using a different machine and I received the same error. I used XO from quick deploy on the main screen. Both of the VMs imported fine using the quick deploy. This was on a fresh install last week. (My First Time Using XCP-ng)
The first VM was smaller than the other two. It was exported out on a Windows 10 machine. The other two VMs were on a Windows Server 2012R2 machine.
I am sure other people have done this before as well, but I used Veeam Agent to import most of my larger VMs. I booted off the ISO and restored from my backups and had little problems with getting the machines to work. The only thing I changed was the NIC type to an intel because I wasn't getting gigabit speeds during the restore.