Upfront editor will not load on one of my multisite sites.

Hello,
On one of my sites, vienna2016.d2conferences.com, the editor gets stuck at "rendering 7%" whenever I try to enter the upfront editor. It works fine on other sites in this multisite install. I have already cleared the cookies and cache in my browerser and reset the upfront cache from the settings. Please let me know what else I can try.
Thanks!