is it possible to get a valid response ?

For any request which starts with /b2b, you're selecting the www.test.de_B2B_80 pool. But you're not changing the URI. Do you want to remove the /b2b string from the URI? If so you can use string range to do this: HTTP::path [string range [HTTP::path] 3 end].

when CLIENT_ACCEPTED {
   persist none
}
when HTTP_REQUEST {
   switch [HTTP::host] {
      www.test.de {
         switch -glob [string tolower [HTTP::path]] {
            "/test.aspx" {
               HTTP::redirect "http://111.111.111.111[HTTP::uri]"
            }
            "/b2b*" {
               persist source_addr
               pool www.test.de_B2B_80

                Remove /b2b from the path
               HTTP::path [string range [HTTP::path] 3 end]
            }
            "/" -
            "/welcome.de.aspx" {
               HTTP::redirect "http://www.test.de/default.aspx?/de-de"
            }
            default {
               persist source_addr
               pool www.test.de_80
            }
         }
      }
   }
}

Aaron