Thanks for checking. That's useful, even if it points away from where I was looking.
Clocks within a second of each other means drift probably isn't your problem, and I'd guess the MST/UTC difference is just how dom0 displays it, though I'm not sure.
What I keep coming back to is that your full backups fail while the delta jobs on the same hosts never do. That's the same split in https://github.com/vatesfr/xen-orchestra/issues/9181, where full backups hit BodyTimeoutError on VMs with big disks or a lot of free space and the deltas are fine.
If your failing VMs look like that, your dates and the MST/UTC detail would do more good on that issue than buried in here.
MajorP93 created this issue in vatesfr/xen-orchestra
open
Full backup fails on large VMs / VMs with lots of free disk space
#9181