empty UNIQID in url parameter confirmation email link

Hi, my subscribers keep getting errors when trying to subscribe through the confirmation link after subscribing via my hustle popup with mailchimp integration. The integration is working ok, the subscriber list is visible in the admin area , just the confirmation link is flawed due to a not filled in variable in the url parameters: e=|UNIQID|. Whole link see below.

https://xxxxx.us10.list-manage.com/subscribe/confirm?u=1d2c9bc44ca62bd32ef77fe06&id=f7b8f312eb&e=|UNIQID|&orig-lang=1

When a subscriber clicks the link in the confirmation email, a white page appears with only the message “Cannot find an email address id”
Field mapping is correct, so that is not it. Google also has no answers. What could be wrong?

Thanks.

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