I am having the exact same error message.
“An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security.”
A few items to help you fix:
• I am on 4.2.2.
• Most times, I am able to get into Customizer and make some changes
• Other times, it gets stuck saving. The circle just keeps spinning and then locks up.
• My speculation is since it’s effecting the Login area, WordPress security is flagging it as suspicious.
• I have to uninstall plugin and then reinstall.
• Seems fine when I reinstall, as long as it doesn’t get stuck again saving.
Love this plugin. I know you worked hard on it and we are very thankful for your contributions to the WP community!
Please keep us updated on future fixes so we can use it again.
ZC