ob_end_flush()

I wouldn't recommend disabling the wp_ob_end_flush_all() function entirely, and I definitely wouldn't turn off zlib.output_compression in your php.ini file. Here's a better approach that replaces the source code causing the issue, and preserves the underlying functionality:

/**
 * Proper ob_end_flush() for all levels
 *
 * This replaces the WordPress `wp_ob_end_flush_all()` function
 * with a replacement that doesn't cause PHP notices.
 */

remove_action( 'shutdown', 'wp_ob_end_flush_all', 1 );
add_action( 'shutdown', function() {
   while ( @ob_end_flush() );
} );