I learned yesterday by direct experience that the USB ports on a laptop can be substandard when running on the laptop's internal battery, and that this can be affected by power-saving settings, which in turn can apparently be reset by Windows updates or driver updates, causing sudden onset of symptoms.
The symptom looked like a hardware failure in my iOptron mount. Either "iOptron Commander cannot find any compatible iOptron devices" or, worse, from Windows, "The USB device you have plugged in is malfunctioning."
It was intermittent. Occasionally I would get a good connection.
The problem ALWAYS went away when I powered the laptop externally.
It also never appeared with my other laptop, which is why I quickly concluded it wasn't the mount.
Other USB devices worked fine with the original laptop, but I didn't try a very great variety of them. It's possible FTDI needs to tweak something in their Windows driver for the USB chip used in iOptron mounts and many other pieces of scientific equipment. But right now the solution is to power the laptop externally, as I always do in the field. (This is an older ASUS laptop with limited battery capacity.)