Patch for upcoming Safari release.
This happens to me every once in a while and it's the only annoying thing that C3 does after using it for a couple years. I can't recall exactly how I've fixed it but I think I logged into the website and then launched C3 from there. After I did that I think I was able to launch it from the downloaded version and login without error. If that doesn't work, a password reset or, as a hail marry, clearing your history cache and cookies. It will be a pain because this will get rid of all of your installed plugins but it might come to that. Hope that helps!