Doesn't seem to work anymore?

Spent some time debugging — looks like you are looking for the Etag from Github and/or Dropbox. But neither site sends the Etag field in their HTTP response headers (when I try it, at least).

If your plugin doesn’t get an etag, then it just stops and puts up a blank error form.

You should use raw url.
You used page designed.

I tested up on my computer using your markdown file. See the screenshot.

Dropbox is same, too. You may use ‘Share Link’ instead of ‘Public Link’. Difference is below:
Public link / Share link
Unfortunately, ‘Public link’ has became paid service.

Thanks.