Just a dumb idea, but have you tried mapping a drive letter to the unc path and then telling move to move to the drive letter. Once the move is complete then you unmap the drive letter. It could be that it just doesn't like something about the UNC path. I appreciate that you said that some UNC paths work but you never know.

I very vaguely remember having problems with the move or delete command in kix some years ago and I shelled the command and used a program - deltree I think! This was not to do with unc, but nevertheless I also had problems using the kix move or delete or something many years ago.

I think it was something to do with the file/directory being locked for some reason.
I have also found copying, then deleting original directory works better than move.