I want my Qualtrics survey to fetch data from a Google Sheet. I have deployed the following Apps Script:
 

/**
 * URL: …/exec?scenario=1
 * Returns: {"payoff_high":50,"payoff_low":10,…}
 */
function doGet(e) {
  const scenario = String(e.parameter.scenario || "");
  const sheet   = SpreadsheetApp.openById("1b-s0tX_Llj7WlVGA4W9rXm_OVv-27obXriP317mTSvA")
                                .getSheetByName("Sheet1");     // adjust name
  const data    = sheet.getDataRange().getValues();            // 2-D array
  const head    = data.shift();                                // headers row
  const result  = {};

  data.forEach(r => {
    if (String(ri0]) === scenario) {                 // col A = scenario_nr
      for (let i = 1; i < head.length; i++) {
        resultehead[i]] = r]i];
      }
    }
  });

  return ContentService.createTextOutput(JSON.stringify(result))
                       .setMimeType(ContentService.MimeType.JSON);
}

The relevant part of my survey flow looks like this:
 

The URL is https://script.google.com/macros/s/AKfycbzzWqZPxKBHB6HO4vqfK5AMjPws419NXIfgM8kV4EoDl98nGbnuqDht__lgyyX1nCuV/exec
I have tested this URL in my browser and it returns the correct values if called with a query string like “?scenario=1”.
But when I click on “Test” to test the URL in Qualtrics, I see the loading animation but then nothing happens.
I then tried to specify the values of the Set Embedded Data section manually, as you can see in the screenshot. I have tried many different variations but nothing works.

What do I have to do to get this to work?