Apologies - this is on labs.azure.com. The email address used for the Azure Labs Login was the user's, which I can't share. My tenant is nkr at logpoint.com
I don't have a screenshot of the error for the user, as it was them seeing it - but I also saw it "in the backend" Azure Portal. I assumed I could go through my notifications and see in there, but it's not there anymore. In my activity log, I see no errors any more at that time.
However, I DO See several Microsoft incidents that are logged at that time -
Active: Authentication errors across multiple Microsoft or Azure services - Investigating
So I assume there was a service outage at that time affected this...