I may have the same issue, I get the same exception (not defined for ExtensibleObject) on these gallery pages:
admin.php?page=nggallery-manage-album
admin.php?page=ngg_other_options
admin.php?page=nggallery-manage-gallery&mode=edit&gid=1&_wpnonce=1510997d30
This last one is when I try to edit any gallery item from the manage gallery overview page.
I think this is a related or the same issue:
https://www.ads-software.com/support/topic/exception-thrown-not-defined-for-extensibleobject
What I did prior to detecting the error was moving my website from a local copy to the live server:
– exported SQL dump from local DB
– changed all references in the sql file from localhost to my website url
– ran sql through a fix-serializiation script (https://github.com/Blogestudio/Fix-Serialization/blob/master/fix-serialization.php)
– uploaded sql dump to live site
– copied everything wordpress to live site
Besides not being able to open some of the options pages (see above), all my menu items on the website link to the home page. I deactivated the plugin for now.
I filed a bug report though I am using the free plugin so not sure how fast or if at all someone from photocrati will get back on that.
Any help from the forums much appreciated.