I'm adding this issue to save other people time. We had views_flag_refresh working perfectly on a site, but at some point as we finished the site views_flag_refresh stopped working. Unfortunately when it stopped working wasn't exactly clear. The flag would still be added, but the block showing the items would start to refresh and never reappear. The javascript error we received was more of a red herring...

Error: xhr is not defined Source File: /sites/all/modules/views_flag_refresh/view... Line: 94

We spent a few hours looking at the version of jquery and other modules that may have added a conflicting script before realizing that is was the Secure Pages module that was causing the problem.

Adding flag/flag/playlist/* to Secure Pages' Ignore pages setting solved the problem.

Thanks for the great work! This is a really slick solution for creating playlists.