Hi Andreas,
The configuration seems to be all right to me. The strange thing is, that I don’t get any results for any keyword, which is very odd. I made a test search to see if I can replicate this, but I am getting results with that.
Do you know if there is any custom code you are using (hooks, filter, template etc..) with the plugin, maybe in your theme functions.php file? I tried to look into it, but the theme editor is not enabled. If not, can you please add temporary FTP access so I can check what is wrong here? Thank you!