Problem with VSS warnings and not finding any files is reproducible on a 100 MB VHD which is a much nicer test platform than my actual C:\ drive. Also, somewhat fewer unwanted files are there.
V:\>dir /ah
Volume in drive V is New Volume
Volume Serial Number is C2C3-E7A6
Directory of V:\
07/03/2024 06:57 AM <DIR> $RECYCLE.BIN
07/05/2024 01:15 PM <DIR> System Volume Information
0 File(s) 0 bytes
2 Dir(s) 85,295,104 bytes free
V:\>dir
Volume in drive V is New Volume
Volume Serial Number is C2C3-E7A6
Directory of V:\
07/05/2024 07:25 AM 1 in_root.txt
1 File(s) 1 bytes
0 Dir(s) 85,295,104 bytes free
V:\>
V:\>"C:\Program Files\Duplicati 2\Duplicati.CommandLine.exe" backup "file://C:\Duplicati Backups\vhd1\\" "V:\\" --backup-name=VHD1 --dbpath="C:\Windows\system32\config\systemprofile\AppData\Local\Duplicati\JCKJYWHHLF.sqlite" --backup-id=DB-4 --encryption-module= --compression-module=zip --dblock-size=50mb --no-encryption=true --snapshot-policy=On --disable-module=console-password-input
Backup started at 7/5/2024 1:37:53 PM
Checking remote backup ...
Listing remote folder ...
Scanning local files ...
0 files need to be examined (0 bytes)
Error reported while accessing file: V:\ => The parameter is incorrect. : '\\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy86\'
Error reported while accessing file: V:\ => The parameter is incorrect. : '\\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy86\'
Checking remote backup ...
Listing remote folder ...
Verifying remote backup ...
Remote backup verification completed
Downloading file duplicati-20240705T170526Z.dlist.zip (455 bytes) ...
Downloading file duplicati-i3d9daaa05e2140d38a1a9acf602639c5.dindex.zip (606 bytes) ...
Downloading file duplicati-b73a1f586e1a24bcca0ddb0cd8b121176.dblock.zip (1000 bytes) ...
Duration of backup: 00:00:07
Remote files: 10
Remote size: 11.25 KB
Total remote quota: 930.97 GB
Available remote quota: 42.01 GB
Files added: 0
Files deleted: 0
Files changed: 0
Data uploaded: 0 bytes
Data downloaded: 2.01 KB
Backup completed successfully!
V:\>
The GUI view of a failure is here.