Endless checkout spinner but only with child theme

I am setting up a store. When I use a child theme for the standard twentytwentyone theme, the checkout spinner never resolves.

I have seen this page and followed the instructions which didn’t help.

It all works if I use the parent theme.

I have stripped it back until the only code in functions.php is

// set up parent and child themes
add_action( 'wp_enqueue_scripts', 'enqueue_parent_styles' );
function enqueue_parent_styles() {
   wp_enqueue_style( 'parent-style', get_template_directory_uri().'/style.css' );
}

and the styles.css file is a clone of the parent, but with the header info changed.

Any suggestions on this?

I know I could fall back to the parent theme, but we know the dangers of doing that.