I encountered a similar problem like HumanEclectic – with the plugin stops working customize.php =(
The reason for this work is that part of the script:
function ds_override_load_customizer_action () {
wp_die (__ ( ‘The Customizer is currently disabled.’));
}
When you try to install the plug-in Toggle Meta Boxes (v3.8.1.1) to change the Menus (v4.3) settings site stopped working by generating “500 Internal Server Error”
The Menus plugin toggles all sorts of WordPress menu functions, including the Customizer.
Manage menus, including the Customizer, at the Network->Settings page in the Menu Settings->Enable administration menus section. Add a check next to “Appearance Customize”.
I included the ability to enable/disable the customizer in a recent update.
Toggle Meta Boxes (v3.8.1.1) has not stopped working in my installs, nor does in generate error for me in concert with the Menus plugin. A more descriptive step by step report is needed for me to reproduce your problems with it.