Yes, same here. It should be easy to trace for author as they should work on plugins with debug mode ON. Above solution should be added to plugin. Anyway this constant is used in wp-includes/default-constants.php
if ( !defined('PLUGINDIR') )
define( 'PLUGINDIR', 'wp-content/plugins' ); // Relative to ABSPATH. For back compat.
Fixing this would enable work with plugin when debug mode is ON, cause now above Warning produces other
Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent
Keep up great work!