Hi, I had a similar problem with a clients site. After being warned about the upgrade locking users out, they deactivated the plugin. The result of this was the following code was put in wp-config.php
define( 'FORCE_SSL_LOGIN', true );
define( 'FORCE_SSL_ADMIN', true );
define( 'DISALLOW_FILE_EDIT', true );
Deleting or commenting out this code solved the problem.
Cheers