I checked source  after reading Jarrels note about the missing function "json_lasr_error" and tought we had a typo, but this is clearly not the case. Now the function "json_last_error()" was introduced to PHP with version 5 .
Could you please check what PHP version you got? Simply install the following plugin or check your admin account at your hoster (or ask them). The minimum required version for WordPress itself currently is 5.2.1 and for this plugin it's 5.0.0. So far I think you both just haven't got a current PHP version running.
/* Plugin Name: Show PHP Version after footer */
add_action( 'shutdown', 'wcm_show_php_version' );
if ( version_compare( PHP_VERSION, '5.0.0', '>=' ) )
printf( 'I am using PHP 5, my version is: %s', PHP_VERSION );
Please come back with more info.