In my case, after many hours of testing and analysis, I have been able to detect that there is an incompatibility of Nextgen with jquery-migrate, which since version 5.6 has evolved to jQuery Migrate 3.3.2, generating some incompatibilities and malfunction of plugins that do not they have adapted to change.
The temporary solution has been to install and Enable jQuery Migrate Helper (https://www.ads-software.com/plugins/enable-jquery-migrate-helper/) and configure the plugin in tools / jquery migrate, to a previous version.
And start working properly.
