SorryPage with iFiles on 11.3 HF8

since there is if-statement (HTTP::uri eq /sp) before switch, /logo and /background won't hit the switch-case (because it is not equal to /sp)

Correct. You should just need to remove the if layer:

when HTTP_REQUEST {  
    log local0. "HTTP_REQ: Requested Hostname: [HTTP::host] URI: [HTTP::uri]"
    switch [HTTP::uri] {
        "/sp" {
            HTTP::respond 200 content [ifile get "index"] "Content-Type" "text/html"
        }
        "/logo" {
            HTTP::respond 200 content [ifile get "logo"] "Content-Type" "image/png"
        }
        "/background" {
            HTTP::respond 200 content [ifile get "background"] "Content-Type" "image/jpeg"
        }
    }
}