E-newsletter templates don't load

First off, this plugin is great. I love it.
I am having small annoying technical problem though.
On the Newsletters list screen when I click to create a new newsletter or edit an existing one, it loads a blank page. Url looks like this:
/customize.php?wp_customize=on&theme=Simplicitylight&newsletter_id=5&return=admin.php%3Fpage%3Dnewsletters
The page loaded has empty html, head and body tags, and the console does not show any errors or any information whatsoever (in chrome, anyway), so the loading process must be shutting down really early somewhere.

Could this be a javascript conflict? I tried deleting the enqueued scripts one by one from the e-newsletter.php but it did not change anything. I will try testing with disabling all addons tonight, but if you have any ideas, please let me know.