Open .xlsm files

Is it possible to back up open files such as MS Excel files (file extension .xlsm)? Currently, I have to close these types of files as I will get a “Warning”:

2024-03-15 19:14:09 -04 - [Warning-Duplicati.Library.Main.Operation.Backup.FileBlockProcessor.FileEntry-PathProcessingFailed]: Failed to process path: D:\Checking & Passwords~$Checking.xlsm

Yes but Windows rules and Duplicati install limitations make it somewhat awkward to get this.

snapshot-policy is how one gets Windows VSS snapshots to allow reads of such locked files:

This setting controls the usage of snapshots, which allows Duplicati to backup files that are locked by other programs. If this is set to off, Duplicati will not attempt to create a disk snapshot. Setting this to auto makes Duplicati attempt to create a snapshot, and fail silently if that was not allowed or supported. A setting of on will also make Duplicati attempt to create a snapshot, but will produce a warning message in the log if it fails. Setting it to required will make Duplicati abort the backup if the snapshot creation fails. On Windows this uses the Volume Shadow Copy Services (VSS) and requires administrative privileges.

and that last part is awkward. Even an Administrator group account does not get full privileges active all the time, as that’s a risk. Instead Windows pops up a window before they’re enabled.
The UAC user experience has “Do you want to allow this app to make changes to your device”

So one option is to endure the prompt, for example start Duplicati from Start menu search and “Run as administrator” assuming your account type is Administrator, or you have one available.

Another option is right-click of desktop shortcut and use Properties → Shortcut → Advanced to check “Run as administrator” box.

The Internet has some awkward Task Scheduler tricks to elevate, but I won’t get into that option.

To avoid a UAC prompt each time (and get more power), set up Duplicati.WindowsService.exe.
Duplicati Tutorial 02 Install Duplicati as a Service is old, but still pretty close, for an initial install.
Migrating from User to Service install on Windows is a lot of setup, but some is extras, such as getting the TrayIcon back (if you like it). If you don’t care for it, then don’t bother setting that up.