The “Non-existent changeset UUID.” error appears for me in Appearance -> Customize (live editing) when the WordPress Address (URL) and Site Address (URL) settings are not set the same. The error does not occur when they are set the same. This has been reported earlier by other users. Is there a fix? (I need the two Addresses to be different.) If not, is the problem in WordPress or GeneratePress? Thanks for your help.
The same error occurs (and can be toggled on and off) when I use the Twenty Twenty-One theme. I’ll report the bug to wordpress.org; let me know if you have any other suggestions.