This has also happened on several of our sites due to the recent attacks on WordPress: http://forum.ait-pro.com/forums/topic/global-brute-force-attack-on-wordpress-sites/
What is happening is that the log files are getting extremely large very quickly – too quickly for the hourly Cron check to zip and email the log files.
In your specific case your log file was already over the 2MB log file size limit so you will need to follow these steps below to fix the issue/problem.
FTP to your website or use your Web Host Control Panel File Manager and delete the security log file in this folder – /wp-content/bps-backup/logs/http_error_log.txt.
Or if you would like to keep the security log file then download it first to your computer before deleting it.