This morning Costco rolled out a major update to their Photo Center website. The new web site runs a completely new API, which makes all current versions of the Costco Export Plugin non operational. It goes without saying, they never gave me any advance warning.
As of right now, when attempting to authenticate the user, the plugin enters an endless redirect loop.  Please do not attempt using the plugin until the next version has been released. I would need a few days to learn and implement the new API, and cook up an update.

Thank you for your patience and understanding.