White screen after subscribing

Hello,
I am using the free version of the thenewsletterplugin and after someone fills out the form and press on the button then first appears the white screen and after a while the user has been redirected to the redirection page /thank you page

The code on the subscription page setting is: [newsletter_form confirmation_url="https://www.mypage.com/"]

Now my question is how to avoid this white screen to appear, because when the customer clicks on subscribe on my newsletter page, then this page disappears and it looks like a error.

Best regards