Permissions Error

I am getting this error over and over in the error log. Any ideas?

[Fri Mar 28 17:02:26 2014] [error] [client 50.116.16.203] PHP Warning: file_put_contents(/nas/wp/www/cluster-1113/arthro/wp-content/plugins/all-in-one-event-calendar/lib/class-ai1ec-loader-map.php) [function.file-put-contents]: failed to open stream: Permission denied in /nas/wp/www/cluster-1113/arthro/wp-content/plugins/all-in-one-event-calendar/lib/class-ai1ec-loader.php on line 378

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