Cannot activate Mailpoet 3 activation key for free sending plan

I can’t activate the key for a free sending plan in WordPress 4.9.1 + MailPoet 3.3.2.

First error is that the key is not a valid premium plugin. That is correct because it is a free sending plan key. I copied it from the Mailpoet account page.

The second error is that there is a error when the key is validated. The service is not available. Please try again later.

Copy of the Dutch error:
Fout bij valideren MailPoet verzendservice-sleutel, probeer het later nogmaals (Dienst niet beschikbaar)