Themes->Customizer blank

hi.

We are using the iThemes security (free) plugin on our site, which is using Divi (3.28 and up to date) on WP version 5.2.3 (both latest at time of writing). However, when you navigate to Appearance->Customiser inside wp-admin, we are getting a ‘you are customising..’ message but then a blank page. If we deactivate the iThemes security plugin, this page works correctly. I can’t see any errors in the php or apache logs and I can’t see any plugin settings that might be causing the conflict. Other plugins installed that might conflict with iThemes Security (guessing here) are WP Super cache and WP optimise.

Has anyone seen this before and can offer up a fix or workaround?

cheers