White Screen in wp-login.php

I was running WordPress 3.5.2.

I recently upgraded my server from PHP 5.3.x to PHP 5.4.23 and went from MySQL 5.0.96-community to MySQL 5.5.

All was working well UNTIL my login cookie expired. Now, I am unable to login to the Admin panel. I have tried Chrome, Firefox, IE11, and mobile Safari (on iPhone). In all cases, when I submit the login form at wp-login.php enter my username and password, I am greeted with a white screen with a lonely period (.) character.

I have done exhaustive Google searches and have already performed these steps:

• Upgraded to WordPress 3.8.
• Renamed /wp-content/plugins folder to /wp-content/plugins-disabled.
• Renamed my active theme directory from /wp-content/themes/yoo_sphere_wp to /wp-content/themes/yoo_sphere_wp_disabled.
• Checked functions.php and wp-config.php for extraneous spaces. Actually, neither had a closing ?> tag, so I added that in both cases.
• Turned on debugging to a file:

define(‘WP_DEBUG’, true);
error_reporting(E_ERROR | E_WARNING | E_PARSE | ~E_STRICT);
ini_set(‘display_errors’, 1);
@ini_set(‘log_errors’,’On’);
@ini_set(‘error_log’,’phperrors.log’); // path to server-writable log file

• I’ve checked phperrors.log, as well as Apache errors for the domain, as well as PHP’s “error_log” file for the domain. None mention wp-login.php or anything to do with logging into the admin panel.

I am at a loss.

I don’t suppose someone can tell me where this “.” character comes from, so I could put some error logging before WordPress reaches that point? PHP scripts don’t just magically display a period character. It has to come from SOMEWHERE after some degree of failure has been determined.

Is there any way I can add some echo statements or checks to wp-login.php to try to debug this? Any code I add to the file is only displayed when the initial form is shown. wp-login.php doesn’t even seem to be executed when the form is submitted. It doesn’t even get there.