Balsamiq mockups failed to load with 404 error - retrieval of attachment descriptor fails

Summary

On a draft page, adding a Balsamiq Mockups Macro fails and results with a 403

Environment

Replicated on Firefox and Chrome

Steps to Reproduce

1. create a draft page
2. launch the Balsamiq Mockups 3 addon form the "+" menu
3. the add-on is not able to retrieve the attachment descriptor and fails

Expected Results

the add-on is able to retrieve the attachment descriptor

Actual Results

the add-on is not able to retrieve the attachment descriptor and fails

Workaround

Publish the page first prior to inserting the macro

UPDATE

We are receiving reports from customers unable to access attachments descriptors also from pre-existing pages. So it seems that the issue does not affect only DRAFT pages.

From customer: "All pages that we updated recently are affected with this problem. Pages, changed longer ago are working fine."

The workaround proposed (save the page before launching the add-on) is not applicable in this cases.