There is a limitation with iOS' in-app browsers which doesn’t allow .zip file downloads. Because of this, depending on how photos are downloaded, you may receive an error that looks like the following:
This happens because you are using an in-app browser to open the download .zip file (for example, opening the gallery link shared via Facebook). So instead of the HTML that the browser is expecting, it's receiving the .zip file that it cannot read.
To resolve this, copy the original secure link, then switch to a real browser (Chrome/Safari/etc,) and paste the link to open the gallery.
Please note: Don’t just copy the link from the in-app browser, because, at that point, the access token link would have already updated the URL to display the guest/public link.
Also Note: Do not use the "Open in browser" feature in in-app browsers, because it'll just open the guest/public link in the browser as well.