Err_ssl_protocol_error

I published an under-development version of my app to the randomly-generated backendless subdomain: https://manlyshape-us.backendless.app/

It works fine on my desktop and my laptop that I have used for Backendless development, but on multiple other devices it gets an SSL error: ERR_SSL_PROTOCOL_ERROR

The SSL button in settings is checked and I am on a paid account.

This is what I see on my phone: