I received a 502 error when trying to enter a password on a password protected post on a WPEngine site while using your theme. I spoke with their support and they said it was because they have anti-brute force protections in place and code in the theme was trying to bypass this. Their system works by filtering 'site_url', which is how WordPress does it their core code (see here: https://core.trac.wordpress.org/browser/tags/4.9/src/wp-includes/post-template.php#L1625) but your theme doesn't. In the wpbootstrap_password_form function of the parent theme you have the following code (line 266 in functions/bootstrap-wordpress.php):