Mautic API + Ninja Popup Plugin for Wordpress

Hello,

we have a problem with the connection to an API that we created in Mautic. After the instalation of “Ninja Popup Plugin” in Wordpress we try to connect the plugin with Mautic using the “Mailing List Manager” of “Ninja Popup” plugin.

We set this settings:

URL to API: https://mywebsite.com/api
Public Key: that we get from Mautic API
Secret Key that we get from Mautic API

but if we try to connet we get the following error:

Incorrect access token parameters returned: Array
(
[message] => Es sieht so aus, als ob wir auf einen Fehler gestoßen sind (Fehler #400). Bitte melden Sie dies dem Administrator wenn es nochmal passiert. (error is deprecated as of 2.6.0 and will be removed in 3.0. Use the errors array instead.)
[code] => 400
)

What can we do to fix it?