I should probably post again, since there’s some new information that might help if someone else runs into this problem. Last time I posted I thought the problem was gone, but I was wrong. Anyway, I’m sure it’s gone now. I’ll explain what happened. It’s somewhat complicated.
I started using rclone a few months ago, on three Macs, running big jobs every night. (One job is backing up a 2tb drive with almost 2 million files. The first time, rclone ran continuously for about 3 weeks.)
rclone worked great. Then one day about two weeks ago, the above error suddenly started appearing on all machines. It had never appeared before (and I’m always reviewing the logs carefully). It would appear frequently and prevent jobs from finishing. With your guidance, I looked closely at the router, and ended up doing a factory reset. This fixed the problem on two of the three machines. However, the problem persisted on the third machine. This is weird, because that third machine is running the same OS, the latest Sierra, as one of the other machines.
After many failed attempts at troubleshooting the problem, I finally surrendered and did a complete erase and reinstall on the boot drive. Not that hard to do, and something I like to do every few years, anyway. And now the error is gone.
It seems so strange that the router reset solved the problem on two machines but not the third. I really wanted to identify the specific cause of the problem (on the third machine), but in the end I could not, even though I tried a bunch of different techniques. For example, the problem persisted after I created a new user account, and it also persisted after I reinstalled Sierra (without erasing the prior installation). Although the problem didn’t happen when I booted the same machine from an external drive containing a fresh, new Sierra.
The error was hard to nail down because usually it would appear within 5-10 minutes after launching rclone, but sometimes it would not appear for 30-40 minutes. I noticed that if the error appeared once, it would keep appearing, pretty frequently. I also eventually noticed that if I got through the first hour, then rclone could run for many hours/days and the error would never appear. This is why troubleshooting was a slow process, and why I got fooled earlier when I declared victory prematurely, and why I finally surrendered to a relatively extreme solution.
Anyway, in case this ever happens to someone else I hope they read this and it helps them somehow.