Edit option shows balnk screen

Hi, I am not able to edit an existing gallery and or add a new gallery on a page via the button in the TinyMce editor. It simple displays a white pop-up screen. It was working perfectly up until last week. No new plugins were installed. I thought it might be the latest version of WordPress and reverted back to version 4.1.5 as per notice on the plugin home page, but still the white screen shows…seems like it just loads…..

https://wordpress.org/plugins/nextgen-gallery/