I just tried the Seamless Sign-in feature and discovered that it’s functional again! I never did receive any word from engineering acknowledging the problem existed in the first place, so I can only keep my fingers crossed that it’ll stay “fixed”. But, for now, it works!
I apologize (on Shutterfly’s behalf) for any inconvenience this issue caused, and appreciate all of the trouble reports I received. Please let me know via the Feedback page if you encounter other issues in the future.
-Don