After several phone calls to Shutterfly, and working with four different Level 2 Support agents, I was told today that engineering is aware of the problems with their Seamless Sign-in facility, and that it’s being worked on. I’ve been told to expect a status update soon. When I hear something more, I’ll post it here.
Until Shutterfly fixes the problem, please disable the Shutterfly Upload plug-in’s “Use Seamless Sign-in” option using Lightroom’s Plug-in Manager.