I copied my WPEngine hosted production site to staging then tested Authy on the staging site and it works fine with all plugins enabled.
Next I installed the plugin on my production site, activated it, entered the Authy Production API Key and enabled Authy on the user accounts. I have an Authy Starter account and WordPress application setup in my account. The problem is when I login and enter the iPhone App token it throws a "Error 502 Bad Gateway" page.
Login steps are:
2. Enter the user name and password at the WordPress login dialog.
3. Enter the Authy token at the next screen with the URL:
4. I'm taken to the URL: https://www.mysite.com/wp-login.php with
an "Error 502 Bad Gateway" page from WPEngine.com
To get past the 502 error I have to disable the plugin by renaming the plugin directory via FTP and clear the web browser cache.
I've tried disabling other plugins with no luck. I have an SSL certificate and define('FORCE_SSL_ADMIN', true); set in wp-config.php. The site uses WooThemes Canvas v5.8.1.
Authy is known to work on WPEngine.