Unauthorized. You may remove the customize_messenger_channel param to preview as

This may be a plugin or theme conflict. Please attempt to disable all plugins, and switch to the default Twenty Twenty-One theme. If the problem goes away, enable them one by one to identify the source of the problem.

If you can install plugins, install Health Check. On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.

FWIW, I had this same issue and what fixed it was flushing the cache on the server and removing the NGINX DIRECT DELIVERY option for my site. I’m with Siteground. This was driving me crazy trying to figure it out and did everything on the website backend to try and fix it. This was what did it. I’ve disabled that caching permanently. SG apparently turned this on for EVERYONE without asking first. They moved me over to ‘Site Tools’ instead of cPanel not long ago so this is under Speed/Caching.