If you having problem while importing demo content please follow below steps;

Add below code to the functions.php under your theme folder.

add_filter( 'http_request_args', function( $r, $url ) { $r['headers']['Accept-Encoding'] = 'identity'; return $r; }, 10, 2 );

Reset the failed demo content by using WP Reset plugin : https://wordpress.org/plugins/wp-reset/

Try to import demo content again.