tried it. Same response. But I found your debug code *hehe* I found out that it searched in wp_post so my bad was I had to say it is a multiside. So I changed the multisite Options and dragged the blog to the right side and now it seems to work.
If I have multiple Networking Sites the Search only needs to search the currently used blog. In the Multisite Options is a Notice: If you not choose any site, then the currently active blog will be used! Also, you can use the same search on multiple blogs!
If this is true then why it choosed wp_post instead of wp_3_post (in my case)