@MichaelEischer, thanks very much! You and @rawtaz were exactly right—it clearly is an ISP problem. I just tried backing up at the office and everything worked as usual. My ISP is Spectrum, so I guess I’ll have to start googling around and see what kind of firewall settings I can toggle. I’ll update the thread again when I find the right switch to flip in the app or on the phone with them.
Thank you both so much again for your help with this! I am very relieved to have my backups functional again and will try to provide the most useful postmortem I can for future users shortly.