Hello,
Thanks for your elements, we process some analysis, and it seems that your current issue come from your custom email (Order status notification).
As mentioned by Nicolas, in your email your download link need to have a token, we didn't see it in your custom email.
Now to solve your issue, Save on your side your custom code AND remove all custom parts (Html customization, text customization & Preload customization).
And process a new test, if that solve your issue, then this means maybe that your custom come from a too older email code maybe.
And so, unfortunately maybe you have to re-created it from a more recent version.
Hope this will help you to progress on your subject.
Regards