500 Server Error – users.php

One of the users on my site was trying to update a bulk list of all users, and now the page will not load at all. I am getting a 500 Server Error, debug info shows the following:

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 72 bytes) in /wp-includes/wp-db.php on line 1548

I am unable to locate anything specific in that line of code, but have tried suggestions to increase the memory including editing the .htaccess file, php.ini, and define(‘WP_MEMORY_LIMIT’, ‘1024M’);

None of these solutions have worked. I also tried finding a database table suggested here: http://wordpress.org/support/topic/set-users-per-page-to-999-now-i-get-internal-server-error?replies=5
but I was unable to locate any meta key with users_per_page as suggested

Everything else on the site works fine, it is only the All Users page at wp-admin/users.php which throws the 500 error.

Any help is greatly appreciated!