Very strange. I had restarted twice with no effect. I went to start it in safe mode, pressed down the Shift key until I saw the logo. When it finished rebooting, the problem was solved, but everything looked pretty normal so I checked and it turns out I was no longer in Safe Mode....in fact, I'm questioning whether I ever was. Regardless, the problem is fixed, and so is the problem with Screenshot not working properly. In looking back, I think the problem started when I did a screenshot and moved the small preview to the desktop before it had time to move to the Screenshot folder. I've done that hundreds of times, but this time the image of the screenshot on the desktop couldn't be moved or opened. For some reason that threw tons of things off kilter, including Time Machine...but I have no idea how it actually fixed itself after two attempts to restart didn't do anything. I'm sure there's an explanation somewhere that is way over my paygrade, but the problem is solved.