New photos uploaded to NextGen Gallery 2.0.66 don't display thumbnails

Recently updated WordPress and NextGen Gallery. Uploaded photos do not display thumbnails. Shows missing file icon, but when icon is clicked it displays image. Same thing when “edit thumbnail” is clicked. Thumbnail file is listed in directory but missing file icon appears, not image preview, so it appears path is correct, just won’t show image. Previous images in gallery still display properly.Except when I uploaded previously uploaded file, same thing happened. Then deleted new file but prior file lost the thumbnail also. Any ideas how to resolve?