For readers having similar problems, note a clarification of the fix given by Register Plus (on its Settings page):
If the first line of your wp_login.php is:
<?php
Then the way to add the code <?php session_start(); ?>
to the top line of the wp_login.php file is to change the first line to:
<?php session_start();
(This worked for me, but I don’t know much PHP.)
