Are you certain that you have a local copy on a local drive? Can you check some files using Explorer?
It’s not a local file if the Status column shows a cloud. A green check mark means file is actually local.
You can also choose some file to right-click → Properties → Details to check Availability status.
On a Windows 10 desktop with Duplicati SYSTEM service, backing up OneDrive Personal, cloud fails.
Save disk space with OneDrive Files On-Demand for Windows 10 gives the icons and some settings.
If I actually make the files local, backup works. On cloud file, I get the same “Cloud file access refused”.
Web search shows a lot on this error. Many backup programs say make file local, so I’ll also suggest it.
You might want to test a folder to see if it helps things. Do a right-click Always keep on this device.
Solution! Can’t backup OneDrive files to external HDD shows the Files on Demand box uncheck, if the interface works the same way in your RDP usage. If not, there’s probably some way to get it turned off.
Does each user have a personal OneDrive area, or is there somehow a shared OneDrive they all get?
Somewhat oddly, if I access the file from a Command Prompt (e.g. by type), I can force it to download.
This changes its Explorer Status. I wonder if Windows Server Backup forced downloads also do this?
There’s also a chance (unless you checked backup contents) that it skips files that aren’t actually local.