Same problem for me as well – followed fix offered below – but with no success.
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I can’t get into my admin panel after updating. How do I fix it?
Go to /wp-content/plugins/wordpress-https/wordpress-https.php and uncomment (remove the two forward slashes before) the line below, or go to your wp-config.php file and add this line. Hit any page on your site, and then remove it or comment it out again.
define(‘WPHTTPS_RESET’, true);