Still having no luck with uploading big files. But a support person was quick to reply via email and direct me to the notice shown above. This is the first real outage I’ve had with them. The problem is with the network between Phoenix and LA, where there are ongoing power problems.
Usually when this sort of thing happens it affects everything, but in this case only File Transfer Protocol is broken. It makes no sense that only FTP would break, so my guess is they disabled it intentionally across-the-board, to allow the other services to continue working.