The issue is with the last function in /plugins/advanced-custom-fields-pro/pro/blocks.php, which calls for functions ClassicPress does not have. I put this into an mu-plugin on my WebJournal, but it could be added to the plugins folder. If I run into other plugins that need shims not covered here, I will add them here.
if (!defined('ABSPATH')) die();
if ( ! function_exists( 'wp_body_open' ) ) {
function wp_body_open() {
do_action( 'wp_body_open' );
}
}
//ACF Pro
if ( ! function_exists( 'has_blocks' ) ) {
function has_blocks($content) {
return $content;
}
}
if ( ! function_exists( 'parse_blocks' ) ) {
function parse_blocks($content) {
return $content;
}
}
if ( ! function_exists( 'do_blocks' ) ) {
function do_blocks($content) {
return $content;
}
} 
