News User with Problems

Hello Peoples

I’m from switzerland and i hope you understand what im write

I search a new system for my newsletter and will try MailPoet 3 Beta.
Install is no Problem, activate too.
but when i go to the options and its not relevant i make a change or go direct to save, then i become a white screen

I repeat, i am open the options, scroll down to save, klick on it and the browser screen is white. i can wait one our, it remains white.
i try this with the browsers maxton, opera and edge from win10
the cache is cleared from the browser

Wordpress ist actually, php Version ist 7.1.5

whats going wrong?

greez Roger