Last Logins page Blank

If the page shows the interface correctly but the list of logs is empty it means that someone reset the logs. In the other hand if the page is completely blank (or grey as you mentioned) it means that the PHP interpreter stopped the execution of the script because it reached the memory limit configured for your site, this usually happens when a PHP script tries to allocated too much memory which in this case may happen because the log file that holds the information of the failed logins is too big.

Go to the “Scanner Settings” panel located in the plugin’ settings page check the size of the “failed login logs”, if it is too big I recommend you to reset the file.

If you are seeing too many failed logins in the “Audit Logs” panel it means that your site is being victim of a “Password Guessing Attack” [1], and you may want to either install a captcha in the login page, limit the number of failed logins per hour, or install a firewall. Sucuri offers all these features in the Web Application Firewall – CloudProxy [2].

[1] https://kb.sucuri.net/definitions/attacks/brute-force/password-guessing
[2] https://sucuri.net/website-firewall/

Memory limit does not seem to be the Problem with v1.7.13 and WP 3.4.1

define(‘WP_MAX_MEMORY_LIMIT’, ‘256M’);
define(‘WP_MEMORY_LIMIT’, ‘256M’);

Reset core integrity logs 94.00B of data
Reset last login logs 15.38K of data
Reset failed login logs 4.04K of data
Reset sitecheck logs 4.81K of data

even if I clear the last logins file the page still keeps blank.