For anyone else landing here with WP Engine + Tailor troubles, the issue is WP Engine’s blocking of the WP Heartbeat API. Contact WP Engine support and ask them to unblock the API so you can use Tailor, which should be very quick to do.
An alternative DIY option is to install the plugin https://github.com/PrysPlugins/WPE-Allow-Heartbeat (must upload it yourself, not available through the WordPress admin UI) and try to add the Tailor editor URLs to the whitelist it provides, which should also enable Heartbeat. I haven’t tested that.
I’ve opened an issue in the Tailor GitHub repository with these details as well.
