The path is not needed in this case. The path is in a constant Url within the plugin, and should be correct since it is used and the definition did not fail. The line of offending code is:
We can rule out other path errors since wordpress is up and running.
if ( ! defined( 'WP_CONTENT_URL' ) )
define( 'WP_CONTENT_URL', get_option( 'siteurl' ) . '/wp-content' );
if ( ! defined( 'WP_CONTENT_DIR' ) )
define( 'WP_CONTENT_DIR', ABSPATH . 'wp-content' );
if ( ! defined( 'WP_PLUGIN_URL' ) )
define( 'WP_PLUGIN_URL', WP_CONTENT_URL. '/plugins' );
if ( ! defined( 'WP_PLUGIN_DIR' ) )
define( 'WP_PLUGIN_DIR', WP_CONTENT_DIR . '/plugins' );
require_once( WP_PLUGIN_DIR . '/all-in-one-seo-pack/aioseop.class.php');
The path and filename in the error looks valid. So the file must be missing.
I would delete the plugin and reinstall it.