But the man issue is that I get an “critical error of WordPress” if somebody tries to subscribe to the newsletter. Also the “Mailing test” in the Backend doesn’t work and I get there a critical error. Next to “Send a test message” there is a red button “KO”. How can I fix it?
If you have a fatal error by WP, it is usually logged in the server error logs. WP usually sends an email to the admin address with the error details, but probably in your case is the email action triggering the fatal condition. Check the error logs (possibly ask the provider how to access them).