For WordPress installations that run into the error “Non-Existent Changeset UUID” when previewing Appearance -> Customisation, the solution is quite simple.
There are two possible causes:
1. It is caused by HSTS (HTTP Strict Transport Security) with subdomains such as WWW turned on and having two seperate URLs for SITEURL and HOME WordPress options.
2. It is simply having the plugin WP-Super Cache active. Deactivate it to view preview.
Hope this helps those that run into the issue.