Thanks so much for those suggestions.

Tahir: It indeed might be a group policy, but my googling has not yet turned up its name. I have seen the group policy setting to block the remote clipboard entirely, but I'm not affected by that, since I can cut and paste text just fine.

Larry: Dragging and dropping files used to work in the olden days of RDP, but it's been years since I've been able to use that. I don't know why they removed it, but yeah, I remember it used to be very convenient. In any case, nowadays drag/drop doesn't work between local and remote, even when I'm connecting to computers where copying and pasting works. For example, I connect to some other remote computer, one which doesn't have the problem. I can right-click and copy a file from my local computer and paste it to the remote RDP PC just fine, but I can't drag and drop the file there.