Software Update doesn't take passwords

I'm using Monterey 12.6.3 on a 24" iMac M1 2021 machine. Sys Prefs says that 12.6.4 is available. I click "Install Now", click Agree to the SLA, and then it asks me for "password for the selected user". That's me. I enter my password, and it refuses to accept it. Whaa? I make Administrator the selected user, enter the Admin password, and it refuses to accept it. Whaaa? I log out, and log back in with my user password. Works fine. I know my password. I log out, and log back in as Administrator with my Administrator password. Works fine. I know my Admin password. Sys Prefs Software Update repeatedly refuses to take my password. I reboot. Same thing. What's going on???

Now,I tried sudo softwareupdate --all --install and that seems to download the update, but when it says it's done, Terminal hangs. I reboot,and I'm still on 12.6.3! I try it again, and now it says "No updates available." I'm stuck!

I've seen this complaint posted several places, and there are no obvious fixes. Someone says to reset the SMC which, of course, you can't do with an M1 Mac, except by restarting, which I did.