thanks for your reply.
I'm using the latest version. I have added a 301 redirect in my htaccess to redirect urls with empty ?s= to the source url – now google gets a 301 instead a soft 404.
I hope this helps.
I don't know how this could happen. I just want you to know I have soft 404 messages because of empty ?s=
So I think google will find the ?s= but I don't know where it will come from.
I did the fix with the 301 redirect and now I will see if this helps.
I will reply as soon as google will crawl these urls again and reports again a soft 404.
Many thanks for your help.