Whilst waiting for tech support and dev to get back to me, I tried again and seems the issue maybe also be related to RDI/KIS (Kaspersky) incompatibility rather than just Outlook as I had originally thought. Shutting down Outlook and KIS, the backup succeeds, immediately prior I had two failed backups with Outlook shutdown and KIS running. Has anyone else experienced anything similar?
Now I know how to consistently reproduce the KIS side of the issue, I had run Perfectdisk to defragment my disk and boot time to defrag the system files. Seems each time the defragment process is run, the issue happens every backup until some number of days pass and things settle back to only Outlook needing to be shutdown.