Your Mautic API Key is not valid

Hi,

I just installed Fluent Forms and the Mautic integration add in. I created an API key in Mautic and added it to the integration. When I save the details I am redirected to Mauic asking me if I accept. Although I confirmed and the details are saved, it still shows me “Your Mautic API Key is not valid”. I tried with different keys, reinstalled the plugins in WordPress but all with the same result. I am using Mautic V3.1.0.

What is wrong?