I think you have this error already logged in ~2016 and apparently resolved but now it's back for me and have tried various troubleshooting steps like ensuring that the ssh-agent command runs for various users in different shells and terminals, but can't reproduce the error anywhere else.

Specifically, this happens if my agent is OpenSSH (not pageant), and when Git Version is set to System (not embedded). Switching away either of those doesn't throw anymore.