Thirty to forty seconds of clock drift after a power outage, and then the backup goes through once you fix the time. That could be coincidence, but it might not be.
I don't know enough about how the XAPI client handles time skew to say whether it would surface as a BodyTimeoutError rather than an auth or TLS failure, so I could easily be wrong here.
If it fails again, could you look at what the host clock is doing before you correct it? @pierrebrunet from the XO team is already on this one, and a second data point either way would tell him whether it's worth pulling on. Thanks for putting the logs on the pastebin, much easier to read that way.