[Fixed] New method, error when sending email

Hi guys,
I have installed a plugin called YITH Woocommerce Request A Quote and it is almost fully functional on my website.
Everything works except for the following error that pops up when you press the “Send request” button:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, class ‘WC_Emails’ does not have a method ‘send_transactional_email’ in
/public_html/wp-includes/plugin.php on line 505

I have tried a lot of things to fix this but it just won’t work. It is weird because I have another contact form on my website which works just fine.
Here is a screenshot of the related lines in plugin.php: http://postimg.org/image/eng5eru39/

Here it is in text if you can’t see the screenshot:

do {
      foreach ( (array) current($wp_filter[$tag]) as $the_ )
         if ( !is_null($the_['function']) )
            call_user_func_array($the_['function'], array_slice($args, 0, (int) $the_['accepted_args']));
   } while ( next($wp_filter[$tag]) !== false );
   array_pop($wp_current_filter);
}

I really hope someone can help, this has really been bugging me.
Thanks in advance.