"Mixed Content" and "ERR_SSL_PROTOCOL_ERROR" errors while trying to use SSL/HTTPS with angularjs http GET

Question

I first implemented my site on a remote VPS without SSL, but then recently got certs from letsencrypt.org.

The site is now accessible at https://www.example.com but now all of my angularjs get requests:

var working_url = "http://www.example.com:3000";
$http.get(working_url+"/example01/").then(function(response){...});
$http.get(working_url+"/example02/").then(function(response){...});

began throwing this error in the inspection console:

"Mixed Content ... the content must be served over HTTPS."

I tried switching working_url from

var working_url = "http://www.example.com:3000";

to

var working_url = "https://www.example.com:3000";

and then got this error in the console:

"net::ERR_SSL_PROTOCOL_ERROR"

As per the accepted answer to this question here, I tried adding a trailing '/' to all of my get URLs, but that didn't help.

I also tried @Kareem Elshahawy's solution here, but that just throws "Error: [$http:badreq]"

What am I missing here?

I'm pretty new to all of this, so any help or insights would be greatly appreciated! Thanks!