Error 502 Bad Gateway

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:
1. https://www.mysite.com/wp-admin
2. Enter the user name and password at the WordPress login dialog.
3. Enter the Authy token at the next screen with the URL:
https://www.mysite.com/wp-login.php?wpe-login=mysite
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.

Thanks

https://wordpress.org/plugins/authy-two-factor-authentication/