Cheatin' uh in E-Newsletter with NextGen Gallery

Whenever you try to create new newsletter or edit one, the "Cheating' uh?" error comes up. This usually happens "right away" but sometimes only after you try to make some change or just save the newsletter.

A conflict test showed up that it's related to NextGen Gallery. Even with all the plugins disabled and only Twenty Sixteen theme, E-Newsletter and NextGen Gallery active - that happens. Disabling NGG solves the problem.

I couldn't replicate that on my test setup but could on member's site.

Full credentials available in chat note.