Having spent the last couple of days debugging NextGen, I believe the problem to be located in the source file, in wp-content/plugins/nextgen-gallery/non_pope/class.photocrati_installer.php.
One question: when you go to the "Gallery Settings" page, on the dashboard, do you also see a blank page? If so, what the problem "might" be is that the "display types" posts have been wiped out for whatever reason. If that is the problem, then NextGen must be told to re-create these lost posts.
In the "update" method of the above file, one of the parameters is $reset, which, by default, is set to FALSE. You need to make the method run with $rest set to TRUE. Unfortunately, I have not seen anywhere in the code where you can trigger the method with that TRUE value.
What you can do, if you have "edit" access to the source files on your web host is to make a manual change in the "update" method.
What I would suggest is the following:
- first and foremost, backup your site (!)
- then copy the original wp-content/plugins/nextgen-gallery/non_pope/class.photocrati_installer.php file "somewhere" safe (a directory outside of your document root would do)
- in the wp-content/plugins/nextgen-gallery/non_pope/class.photocrati_installer.php file, locate the line of code (should be line number 122 or there about) that says: $last_module_list = self::_get_last_module_list($reset);
- Just before that line, include a line that says $reset = TRUE;
- Then deactivate and reactivate NextGen Gallery
- Reinstate the original version of the wp-content/plugins/nextgen-gallery/non_pope/class.photocrati_installer.php file from where you copied it.
In theory, this should trigger the NextGen installer to recreate the Display Types posts (which is the info that is shown on both the Attach to Post dialog as well as on the Settings page).
Hope this does help.