Non-existent changeset UUID

This is the current issue when we click on Appearance–>Customize

Hello @magna,

Can you please try to temporarily deactivate all your plugins except Sydney Toolbox and Page Builder by SiteOrigin, and see if it helps?

Please remember about importance of regular full site backups (files and database):
https://codex.wordpress.org/WordPress_Backups

Also you might want to check this tutorial:

Kind Regards, Roman.