Aweber systemically requires the source app sending to their lists (in this case Wishloop) to use the double opt-in system when using the API integration
As Wishloop - and most other similar apps - use single optins, this causes conflict.
To solve this, simply integrate Aweber using the HTML Code route instead of the API route. Instructions on how to do this can be found here
Remember if you change to HTML you will also have to change your individual optin forms on each campaign
Alternately, Aweber will consider allowing users to change to a single opt-in system on a case-by-case basis
if you want to have your account and your lists enabled for this, all you have to do is contact AWeber through their online form, or email firstname.lastname@example.org. Tell them you want to enable single opt-in for your lists for use through their API. AWeber will then either approve or deny your request after they review your account.