Notice: has_cap was called with an argument that is

SOLUTION:

=============== 1) TO PLUGIN AUTHOR =================

You should change the number 8 to administrator in this line:

add_submenu_page('options-general.php', 'WP Slideshow Posts', 'WP Slideshow Posts', 8, __FILE__, 'WPSP_Options_Page');

=============== 2) TO USERS =================

till the problem is fixed, dont touch the plugin… just use this code in your functions.php (or somewhere):

add_action('admin_menu', 'REPLACEE_8888',9); function REPLACEE_8888(){
	remove_action('admin_menu', 'WPSP_Add_Option_Menu');
	add_submenu_page('options-general.php', 'WP Slideshow Posts', 'WP Slideshow Posts', "administrator", ABSPATH. 'wp-content/plugins/wp_slideshow_posts/wp-slideshow-posts.php', 'WPSP_Options_Page');
}