I use the Wordpress 5.0.2 and the plugin Newsletter2go 4.0.07 from here https://www.newsletter2go.de/features/wordpress-newsletter-plugin/
I activated it by connecting it to my Newsletter2Go account and placed the widget in the Sidebar. If I am logged in, all is running fine. I can see the form and can do the newsletter registration. But logged out users do not see any content inside the widget. Only the title is visible. I can see in the developer bar of my browser, that the form-meta-data (json) was fetched from newsletter2go. All seems correct only the form is not rendered.
Do you have any idea, what I can do?
Ok, found it. This feature of wpfastestcache plugin was enabled for not logged in users. https://www.wpfastestcache.com/premium/render-blocking-js/ After switching it off, all was working fine.