This error refers to the execution of a built-in PHP function ob_start() inside a callback which is illegal as described here [1] however, there is no reference to this function in any of the files associated to the last logins page, all of these references are either in the audit logs, firewall, post-hack and malware scan pages.
My guess is that something else in your website is trying to capture the buffer output prematurely, some cache plugins do this so if you are using one it may be a good idea to test again with that plugin disabled. Someone posted this [2] in the development repository of the WP-Super-Cache plugin, if you are using that you could have the culprit now.
Marking as resolved (or more like wont-fix as there is not enough information to determine if the bug is in the Sucuri plugin’s code or not) feel free to re-open if you need more information.
[1] http://php.net/ob_start
[2] https://github.com/Automattic/wp-super-cache/issues/126
