Cannot authorize PC with iTunes even after deauthorizing - it keeps saying "You have already authorized 5 computers with this Apple ID"

1. Recently I had 5 (current and past) devices authorized on iTunes, including a Windows 10 PC. These were all authorized more than a year ago (much longer than the 90-day change limit).
2. They all worked fine a month ago.
3. One day, my iTunes on Windows said it was not authorized and that I needed to authorize. I don't know why, but my guess is that after a Windows update, iTunes no longer recognizes the computer as being the same as before.
4. When I tried to authorize this instance of iTunes, I was told: "You cannot authorize more than 5 computers. You have already authorized 5 computers with this Apple ID. To authorize this computer you must first deauthorize one of the other computers."
5. I go to iTunes -> Account -> View My Account -> Manage Devices and de-authorize all devices that I am not currently using. Now I am down to 2 (my iPad and an old MacBook).
6. I try to authorize my computer again and get the same error message: "You cannot authorize more than 5 computers. You have already authorized 5 computers with this Apple ID. To authorize this computer you must first deauthorize one of the other computers."
7. I figured that maybe there was a delay on the server and that the problem would soon go away.
8. That was at least a week ago. I still get the same error message.
9. I have iTunes 12.12.4 and no new updates available.

Is this a known problem? Is there any solution?

Thanks in advance!