Caught up with what's happening. Annoying when they push changes out, without any prior notice or message on social media. Would hate to be next in line at the Front desk and stay details fail to load.

They were migrating their database, my guess is merging Starwood booking numbers -> Marriott's numbers.
My Upcoming Stays are back now with new confirmation numbers.

People who lack the the Sign-in button on the menu should clear Website Data for marriott.com.