Error 500 in customizer

Hi Tom,

Weird problem: On all my sites using GP Premium, the Customizer suddenly doesn't work--goes to a 500 error. I've tried bumping up the memory to 128MB in WP-CONFIG, but no luck.

The strangest thing is, I really haven't done any updates lately on these. Seems the problem has come out of nowhere.

I'm using GP Premium 1.2.91, Generate Press 1.3.06, and WordPress 4.6.1

Thanks!