I've got the same problem. It appeared after the last plugin update.
This bug is affecting not only Grand FlaGallery, but other plugins that uses External Media and Playlists, like JWPlayer. Now I cannot insert Videos via External Media or Audio Playlists.
I temporarily deactived the Grand Flash Album Gallery plugin to check if it was this plugin that was causing the problem. And things went back to normal. I reactivated the plugin and the bug reappeared.
I need urgently to add new posts and insert videos via External Media and this bug is blocking the media manager. But if a deactivate FlaGallery most of my posts will stop working.
Is there any way to go back to the version 2.52?