Widget Upload to CRM Fails with “Page Not Found” – Even with Correct Index Path

I'm building a simple widget to export Contact data to CSV in Zoho CRM, triggered via a custom button. The widget uploads cleanly, appears in the widget list, and is successfully assigned to a Contact detail view via a custom button. But when clicked, it displays a “Page Not Found” error instead of loading the index.html.

ZIP File Structure (Flat ZIP):

files.zip
├── index.html
├── script.js
└── manifest.json

manifest.json:

json

{
  "platform": "CRM",
  "widgetType": "web",
  "name": "Contact CSV Export",
  "location": {
    "Contacts": {
      "button": {
        "label": "Download CSV"
      }
    }
  },
  "url": "/index.html",
  "version": "1.0"
}

index.html:

html

<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <title>Download Contact CSV</title>
</head>
<body>
  <button onclick="downloadCSV()">Download CSV</button>
  <script src="script.js"></script>
</body>
</html>

script.js:

javascript

function downloadCSV() {
  ZOHO.CRM.API.getRecord({ Entity: "Contacts", RecordID: recordId }).then(function(data) {
    let contact = data.data[0];
    let csvContent = `ID,First Name,Last Name,Email\n${contact.id},${contact.First_Name},${contact.Last_Name},${contact.Email}`;
    let blob = new Blob([csvContent], { type: 'text/csv;charset=utf-8;' });
    let link = document.createElement("a");
    link.href = URL.createObjectURL(blob);
    link.download = "contact.csv";
    link.click();
  });
}

let recordId = null;

ZOHO.embeddedApp.on("PageLoad", function(data) {
  recordId = data.EntityId[0];
  console.log("Contact ID loaded:", recordId);
});

ZOHO.embeddedApp.init();

What I've Tried:

Ensured ZIP includes only the three required files with no nested folders.
Manifest references /index.html (with leading slash, as required).
Deleted and recreated the widget and associated button.
Reuploaded ZIP multiple times via Developer Hub.
Cleared browser cache and logged out/in to reset possible caching issues.

Still getting “Page Not Found” when clicking the custom button from the Contact detail page.

Help Requested:

What else can I check to ensure Zoho loads index.html correctly? Is there a deployment or validation step I’m missing—even after successful upload?

Any guidance or proven examples would be hugely appreciated.