It’s interesting that it jumped straight to an error (or maybe the warning was hard to find). –snapshot-policy:
On Windows this uses the Volume Shadow Copy Services (VSS) and requires administrative privileges.
If the account is a “Standard User”, making it an “Administrator” might solve that, or if you don’t like the idea because of concerns about security or accidents, Duplicati installed as a Windows service uses SYSTEM, which is even more powerful. Migrating from User to Service install on Windows describes the conversion.
I put too much in one paragraph before. One path is to see if the #1400 error can be avoided by requiring a snapshot. That seemingly ran into a lack of permission, per log file. The other path would be to use the log file to catch the details of the #1400 error, including the path, then avoid backing up the troublesome area… For this path, the add-on is only a log file, and snapshot would be left at default of off while chasing #1400.
If you’re able to get the #1400 very reliably, that might be especially interesting in terms of reproducing it so it can be fixed. It’s been somewhat erratic so far, and a nice test case could be a great aid to development.