UPDATE: Answer below, you can do this with the requestReslet() method.

I am trying to execute a GET action on a Suitelet in the same account. I've done this in the past successfully, but for some reason the same code I used before doesn't work now.

This is my test code:

        var suiteletURL = url.resolveScript({
            scriptId: 'customscript_rl_test',
            deploymentId: 'customdeploy_rl_test',
            returnExternalUrl: false
        });
    
        var response = https.get({url: suiteletURL});

And this is what NetSuite returns:

SS_INVALID_URL
{
   type: "error.SuiteScriptError",
   name: "SSS_INVALID_URL",
   message: "The URL must be a fully qualified HTTPS URL.",
   id: null,
   stack: [
      "createError(N/error)",
      "execute(/SuiteScripts/CSG_SS_Test.js:16)",
      "createError(N/error)"
   ],
   cause: {
      name: "SSS_INVALID_URL",
      message: "The URL must be a fully qualified HTTPS URL."
   },
   notifyOff: false,
   userFacing: true
}