Hi @calbzam,
Yes, the user (me) is registered at www.plasticscm.com with the Unity ID. When signing in through www.plasticscm.com, I use the "Sign in with Unity ID" feature successfully. However I am not a paid user -- I am using the Free tier of the Cloud edition.
I would guess the issue has to do with Plastic's command line interacts with the online login, as when it prompts me to log in with an oauth session, the website (which I paste the URL to) says I've signed in but the command line process quits out with the aforementioned error.
I am free to find a meeting time if you would like. I was away this week, hence the late reply, but I am back now and can assist in debugging!
Thanks,
Gabriel