@olivierlambert
I am using xoa
xo-server 5.107.3 Xen Orchestra server
xo-web 5.108.0 Xen Orchestra web client

I think I have exactly the same issue here.
I saw, that xoa was using extremely much RAM memory and then finally crashed, after the disk was exported. wget showed me the message "Error decoding the received TLS packet" during the download. So I increased RAM of the XOA VM up to 22 GB, but still the same issue.

When I read, that there was some NodeJS under the hood, I had the idea, that it could be related to the name of the VM, which I was trying to export. My VM had the name "[XOA Export] Somename" because it had been previously exported from another pool and imported here.

After changing the name into just "Somename" I could successfully export in OVA format.

Maybe this helps you to find and fix the bug?