I think you were right. I just got things working again.
I had cleared all cookies before I emailed, and that had not fixed it, even after quitting and restarting the browser, as well as macosx.
But after reading, I'd come to the same conclusion. I used the reset safari option, but I had to do it twice and reboot macosx again, and THEN the logins worked. I also quit and restarted google drive which was reporting that my cookies weren't turned on, but they were. Some combo of that seems to be what worked.
Thanks.