I had the same problem, and the cause (and fix), was simple.

I had an "ssh" connection (to some server), open in one terminal window (which I'd forgotton about).

While trying to "scp" to the same server in another terminal window (which generated the error).

Closing the "ssh" connection fixed the problem.

Presumably, you can do ONLY ONE "ssh"/"scp" type thing, to any given server, at any given time. Which seems quite reasonable to me.