If nothing will help and you can't import xva with import VM, I have solution.
Deploy new VM with disk x4 times greater than backup file. OS Ubuntu 16.04 or later.
Copy backup. Take attention. You need always copy backup. Do not move it.
Install xva-img https://github.com/eriklax/xva-img
add-apt-repository ppa:ubuntu-toolchain-r/test && apt-get update && apt-get install -y gcc-7
cd to xva-img dir and run next command
Extract xva file
tar -xf my-virtual-machine.xva -C my-virtual-machine
chmod -R 755 my-virtual-machine
In folder my-virtual-machine you will find some directory like Ref :1, but maybe with another number. Remember this number.
Create raw disk from extracted. Replace 1 with number from previous step.
xva-img -p disk-export my-virtual-machine/Ref\:1/ disk.raw
apt install qemu-utils
Convert raw to vhd
qemu-img convert -f raw -O vpc disk.raw [vhd-name-you-like].vhd
Copy vhd to some SR. I'm using local SRs, so in my case path is /var/run/sr-mount/[sr-uuid]
Go to the host, cd to SR folder, get VHD uuid, rename VHD
vhd-util read -p -n [vhd-name-you-like_from-step-8].vhd
mv [vhd-name-you-like_from-step-8].vhd [uuid].vhd
Attach VHD to existing VM or deploy new one
Get you files
wow. I am getting on with this in parallel. Thank you again! Results for both suggestions and requests are coming!
Since the update I have run full backup that have taken over an hour to complete due to their size and many smaller delta backups and the error has not come up again. I assume whatever was causing the error was fixed in the xo-server 5.84.2 update.