Hi Martin,
This works fine on a PC at the location of a client who I support, which runs XP. Also, on my own Windows 10 PC, VSS works perfectly from the VSS application, but when I try to access it from VFP I get "database XXX does not exist".
Sometimes Microsoft's VirtualStore makes it look like a file is in a location, but when you access the same location with a different user or with an elevated application, then suddenly it's not there. The easiest way to check is using MODIFY FILE from VFP to see if VFP has actually the same content that you see from VSS. VirtualStore is located at %LOCALAPPDATA%\VirtualStore.
If it turns out that everything is still OK, I would use Process Monitor as my next step to find out which file location VFP is looking at to find the VSS database and what error message Windows reports back. You can download Process Monitor from https://docs.microsoft.com/en-us/sysinternals/downloads/procmon https://docs.microsoft.com/en-us/sysinternals/downloads/procmon.