Q: WordPress MU Domain Mapping Customizer Error I am using WordPress MU Domain Mapping which works fine, but when I try to these any theme's customizer, I get this error when trying to save:
Cheatin' uh? Sorry, you are not allowed to customize this site.
When I deactivate the plugin, the customizer works. I have 'Remote Login' and ' Redirect administration pages...' unchecked in the options as well.

A: After much trial and error, I found the cause to be a conflict with the WP Super Cache plugin, I had to disable the following from my wp-config.php file: 
define( 'WPCACHEHOME', '/var/www/html/wp-content/plugins/wp-super-cache/' ); //Added by WP-Cache Manager


A: Try mapping without a plugin then. Since WP 4.5 this is possible. Just point the domain to the server with A record and change the site URL by editing the site.
https://codex.wordpress.org/WordPress_Multisite_Domain_Mapping
Or contact the plugin developer here and report the issue: https://wordpress.org/support/plugin/wordpress-mu-domain-mapping
