Yesterday (2021-07-21) I started having a problem with using git against some of my private github repositories. Trying to fetch updates would look like this:
$ git fetch upstream ERROR: Repository not found. fatal: Could not read from remote repository. Please make sure you have the correct access rights and the repository exists.
It turns out this was caused by my using an old 2048-bit rsa key for SSH. Perhaps github are rolling this change out across all private repos. You can create a new 4096-byte key with the command: