Hi Don:
As a Microsoft Certified technician (happily retired now, I might add) I'm not one to go and just do mass cookie deletion. I work around that by trying to find "related" cookies.
However, in this case, even that wasn't necessary.
All I had to do was get to a point where I could do a forced log off from this Microsoft Account, and once I did that, re-logged on and satisfied authentication, all is fine again. Deleting cookies is way too often the suggestion by people when in fact, while that often works, isn't always necessary.
I've been a certified tech (Microsoft, HP, IBM, Lenovo) for decades and I've learned some tricks here and there.
I was caught off guard with THIS particular situation, because with that flash error, it didn't even suggest to people to log off and re-log on. People who don't expect this might not be sure why it's happening and in fact, that message is rather misleading.
If this is due, as I think you suggest, to a "new cookie", the message is poorly composed and doesn't give any indication as to the fix.
If anything, with that message, perhaps Microsoft should send people an email, right then, giving them a link to log off and letting them log in again, fully, with the authentication token being required.
Oh well. it is what it is and I got it working without cookie deletion.