WordPress customizer Preview not loading

(I am allowed to answer but I am not allowed to comment, so, yeah.)

Have you made any changes to the .htaccess or added any .htaccess files?

EDIT:

We might need more information than that.

  • First: Have a look here how to use WP_DEBUG: https://wordpress.org/support/article/debugging-in-wordpress/.
  • Have a look inside wp-config.php, there is a line define( 'WP_DEBUG', false );. Set this to true to turn WP_DEBUG on (and to false to turn it off again).
  • Once WP_DEBUG is on, go to the customizer where it shows either the customizer, or a blank page, or maybe even a PHP error.
  • If there is a PHP error, please post that error here and turn WP_DEBUG off again.
  • If there is no PHP error, have a look inside the page source code, maybe there is a PHP error at the end of the file which is not shown by the browser because of missing HTML closing tags or something. Again, post that error here and turn WP_DEBUG off again.
  • Also (while you have the customizer open) open JavaScript console in the developer tools of your browser, reload the page and check for errors or warnings in there and post the errors here.
  • Do not forget to turn WP_DEBUG off again!

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