copy fails with no error code

[owen-murphy]

A recursive list command for a particular source path works...

>mtpcopy.exe list -R "ONEPLUS A3003:Internal shared storage:\DCIM\Camera"
ONEPLUS A3003:Internal shared storage:\DCIM\Camera
ONEPLUS A3003:Internal shared storage:\DCIM\Camera\IMG_20191203_181652.jpg
ONEPLUS A3003:Internal shared storage:\DCIM\Camera\VID_20210325_151913.mp4
...
...

...but a recursive copy from the same source fails without a good error message:

>mtpcopy.exe copy -R "ONEPLUS A3003:Internal shared storage:\DCIM\Camera" "D:\Pictures\Camera"
ERROR mtpcopy > The text associated with this error code could not be found.

Is there any way to diagnose what's going wrong?
[I'm running mtpcopy on a Windows 10 cmd shell with Administrator privileges]