[Plugin: NextGen Gallery] Cannot edit thumbnail – page not rendering correctly

I have NextGen 1.9.12 (I haven’t had time to do the fairly major update to v.2 yet), but my site is now updated to WP 4.2.1

I find I am unable to edit thumbnails of images in NG galleries. When I click on ‘Edit thumb’, the editing window appears and I can drag the window over the image to set the new thumbnail. However, the entire area of the page, including the edit thumb window, is darkened and inactive. I cannot click the ‘Update’ button, all I can to is Escape to return to the previous screen.

The behaviour is the same in Chrome, Safari and Firefox (all on OS X 10.10).

Has anyone seen the same and maybe found a solution or workaround?