I got it working after the upgrade by editing simply-instagram.php and changing line 14
DEFINE('simply_instagram_plugin_url', get_bloginfo('siteurl') . '/wp-content/plugins/simply-instagram/');
to:
DEFINE('simply_instagram_plugin_url', get_bloginfo('url') . '/wp-content/plugins/simply-instagram/');
and reconnecting my Instagram account via Simply Instagram settings within the admin panel