I have had lots of problems with permissions and roles. It simply disappeared. But I saw a very useful tutorial how to fix it: https://www.role-editor.com/nextgen-gallery-restore-lost-capabilities/
There is no possibility to use this plugin for frontend users. NextGen Uploader does not work for WP 3.9. If you need this, you better choose Nextcellent Gallery.
BUT: In general this gallery is indeed the best one for WP. I would give 5-star rating if the above mentioned problems would be solved.