Thanks, Shonky. Yes, you'd think that the drive redirection feature of RDP is separate from the copy/paste feature. And in fact, simply connecting to a shared hard drive works fine for me, so I have that in place as my functioning workaround.

But I don't know what RDP is actually doing "under the hood" when you copy/paste a file. My symptom is that copy/paste of text works, and it only fails on copy/paste of files, so clearly, it's doing something different for files. Is it creating a brief fileshare or drive redirection to get the job done? Dunno.