Github: "Temporary clone tokens are read-only" error when logging in via HTTPS, but not via SSH

(A close neighbor to this question was asked here.)

I used to use HTTPS to push changes to Github. Now it won't let me. Each time I try, from any of my repos (personal or organizational), I get a 403 error, with the message "Temporary clone tokens are read-only".

Why would Github suddenly decide my clone tokens are read-only? It was working yesterday around noon, and then around 9 pm it wasn't. My configuration was unchanged.

I switched to SSH and the problem went away. SSH is better so I'll keep doing it. But I'd still like to know what went wrong.

Here's an example. I checked out a fresh clone of https://github.com/ofiscal/tax.co, made a branch called ugh, made a tiny change, committed, and tried to push. Here's what happens:

[jeff@jbb-dell:~/of/tax.co/dup]$  git push --set-upstream origin ughremote: Temporary clone tokens are read-only.fatal: unable to access 'https://github.com/ofiscal/tax.co/': The requested URL returned error: 403

I know the problem was not that I was using the wrong password, because when I do that I get a different error:

[jeff@jbb-dell:~/of/tax.co/dup]$  git push --set-upstream origin ughremote: Invalid username or password.fatal: Authentication failed for 'https://github.com/ofiscal/tax.co/'