[Opserty]

Try this...

1. AdminCP > Plugins & Products > Plugin Manager
   
2. From the list look for:
   

   Lucky admin_options_processing [Edit] [Delete]

3. Click [Edit].
   
4. Change the PHP Code (the bit in the big box) to:
   
   PHP Code:

   if (is_array($settings['lucky_group']))
{
    $settings['lucky_group'] = implode(',', $settings['lucky_group']);
} 


It should work, I hope.