Form doesn’t work

I want to use this Newsletter Plugin, because it’s free and has the most features. But I have one problem… The users aren’t getting subscribed to the list…

I already checked the console/network and the problem is that the subscription form doesn’t commit the E-Mail to the post URL…

How can I solve that?

Here is my URL: https://gamekeys-shop.de

You need to check this via incognito tab. Because after you closed/subscribed to the slide windows, it doesn’t appears again (cookie).

Greetings and Thank You!