Error after activating Pro Version

Hello,
I bought Content View Pro, but the plugin does not work. After activation, an error message appears in the front end (see Screenshot under url: https://www.barfuss-schuhe.net/wp-content/uploads/2018/11/Screenshot-Content-View.png)

I have tried the plugin with several themes, everywhere I get the error message.

The problem only appears in the Pro version, but not in the free version.

With the request for help
Best regards
Dirk