I’ve received a report that image uploads using “Seamless Sign-in” have stopped working. I was able to reproduce the problem using Shutterfly’s own developer tools on their web site, making it pretty conclusive that something has changed on the Shutterfly side. I’ve reported the problem to The Black Hole Shutterfly Developer Support.
In the meantime, if you’re having problems, turn off the plug-in’s “Use Seamless Sign-in” option using the Lightroom Plug-in Manager (see the plug-in help for more information). Be sure to reload the plug-in, or restart Lightroom, to activate any configuration changes that you make!