Hello Leo...
I have tried EnableLinkedConnections, but no joy, I'm afraid.
I have proved the button code on paths which did not involve the U: drive, so, I am coming to the conclusion (reluctantly) that RoboCopy has a problem of some sort with the mapping of the U: drive per se.
I am able to address the path using \READYSHARE instead of U:, so, I have tailored the button code to use \READYSHARE_Backups for the source, and I still get the same result.
I am starting to think RoboCopy cannot access the router-connected drive, even though Opus and FileExplorer both can.
Does that idea make any sense to you?