The Pi running the Robotic Arm crashed. When I looked into it, it turned out to be an SD card problem. I replaced the SD card, reloaded the software (I do have backups, of course) and it's working again.
My plan had been to take an image of each SD card, so if this happens, all I have to do is splat the image on a new card. This plan failed, because the old card and the new card weren't exactly the same. So I had to copy files, and that did work.