Next Gen/Atahualpa/WP Super Cache Conflict

I installed WP Super Cache 1.4.8, however the pages were not caching. Page source shows this error:

<!– Page not cached by WP Super Cache. Blank Page. Check output buffer usage by plugins. –>
<!– Page not cached by WP Super Cache. No closing HTML tag. Check your theme. –>

Changed to Twenty Fifteen theme and caching worked fine. Changed back to Atahualpa 3.7.24, deactivated all plugins and went one-by-one. All of my plugins worked with WP Super Cache with Atahualpa except NextGen 2.1.62.

Got any ideas on how to resolve this? I really like the theme and I have a lot of galleries. Trying to improve my pagespeed test so would really like to get this to work.

Thanks.