Error: fatal: repository not found (private repo)

Anyone else getting this? This just started yesterday evening and still getting it. The repository does exist and this flow used to work fine earlier in the day.

...
      - name: "Checkout"
        uses: actions/checkout@v2
        with:
          fetch-depth: 0
      - run: |
          git checkout ${{ github.event.inputs.ref }}

...

 Error: fatal: repository 'https://github.com/bitsofinfo/my-private-repo/' not found
  The process '/usr/bin/git' failed with exit code 128
  Waiting 12 seconds before trying again
  /usr/bin/git -c protocol.version=2 fetch --prune --progress --no-recurse-submodules origin +refs/heads/*:refs/remotes/origin/* +refs/tags/*:refs/tags/*

Start with the actions/checkout@v2 workflow shown in the report and inspect the failing git fetch command for private repositories. Reproduce the checkout flow using the supplied ref and repository context, then identify why an existing private repository is reported as missing. Done means the workflow can fetch the private repository successfully again and the cause is documented or addressed.