There's 2 reasons: XAPI doesn't tell us if the transfer is complete or just cut. We just stop to receive chunks, and after a bit, we decide that's it, there's no more data to send (that's the normal behavior even when it works!) XVAs are hard to validate, depending on various factors. We do a VHD validation (so for delta backup) but XVAs are another thing

So far, I noticed this problem for three VMs. Each time I am trying to perform a backup with zstd compression through XO, the backup is marked as successful, but output XVA file is damaged (incomplete). The backup works fine for all other VMs, and I cannot pinpoint what is special about these faulty VMs.