jQuery $ error when activating plugin

You can try using jquery of PS instead the version of WP by default (because WP is more up-to-date than PS) : using Jquery of PS :

function mytheme_init() {
if ( !is_admin() ) {
wp_deregister_script( 'jquery' );
wp_register_script( 'jquery', '', array(), '1.11.0' );
wp_deregister_style( 'jquery-fancybox' );
wp_register_style( 'jquery-fancybox', '/js/jquery/plugins/fancybox/jquery.fancybox.css' );
wp_deregister_script( 'jquery-fancybox' );
wp_register_script( 'jquery-fancybox', '/js/jquery/plugins/fancybox/jquery.fancybox.js' );

wp_enqueue_script( 'jquery' );
wp_enqueue_style( 'jquery-fancybox' );
wp_enqueue_script( 'jquery-fancybox' );
}
}
add_action( 'init', 'mytheme_init' );