Similar issue with a site running WP 3.6.1 - plugin version 3.8.3
New install on that site, not plugin update.
Scan gets stuck at "Scanned contents of (e.g.) 287 additional files at ... per second"
Have to force kill to end scan
This extract from error log may be relevant
PHP Warning: Invalid argument supplied for foreach() in /home/****/wp-admin/includes/plugin.php on line 1290
PHP Warning: Cannot modify header information - headers already sent by (output started at /home/****/wp-admin/includes/plugin.php:1290) in /home/*****/wp-content/plugins/wordfence/lib/wfUtils.php on line 507
Wonder if this is a blank additional line at end of file issue?
Also noted ajax/cron error in server error logs (500 type error)
/wp-admin/admin-ajax.php?action=wordfence_doScan&isFork=0&cronKey=9a5bb9437a87ee05bc0d0
and several server 500 errors for /wp-admin/admin-ajax.php
