Error 123 with anything beyond a simple backup path

I’ve set up a backup system for a number of computers, using a server and urBackup. The clients are installed and doing image backups just fine.
However, since there are the image backups, and some machines are offsite, I don’t want to do full file backups of the entire drive. I just want to backup up User profile Documents/Pictures/Videos/Downloads, etc. You get the idea.
When I set the backup path as C:, everything works fine. When I set it to something like C:\Users:\Documents, to back up every profile’s Documents folder, I get

Cannot access path to backup: “\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy95\Users:\Documents*” Errorcode: 123 - The filename, directory name, or volume label syntax is incorrect.

in the logs.
According to the built in help, under “How to include files”, I should be able to use a colon : as a wildcard to do every user folder. I tried replacing the colon with an asterisk, and got the same error, but with the same replacement in the path of the error.

I thought I’d try removing the drive letter, since it also has a colon in it, and then I got a similar error, but the path was:

“\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy98\ers:\Documents”

The “Us” was missing from the beginning of Users, but it was definitely there in the settings.

I’m running server 2.5.33, and clients are 2.5.25.

Am I reading this documentation wrong, or is this a bug of some sort?

Editing to add requested info from sticky post:

The clients are running on Windows 10, for the most part, and the server is running on the latest Devuan (a systemd-free Debian derivative), with RAID and ext4 filesystems.