iRule Error - 'Denying user - Redirecting Userid :' found

You might want to put "scan" in a condition:

when HTTP_REQUEST { 
    if { [HTTP::method] == "POST" && [HTTP::path] contains "/bql" } { 
        if {[HTTP::header "Content-Length"] > 5242880} { 
            HTTP::collect 5242880 
        } else { 
            HTTP::collect [HTTP::header Content-Length] 
        }
        log local0. "[HTTP::uri] Content Length is [HTTP::header Content-Length]"
    }   
}   

when HTTP_REQUEST_DATA { 
    log local0. "[HTTP::uri]" 
    log local0. "[HTTP::payload]" 
    set parameters [HTTP::payload] 

    foreach parameter [split $parameters &] { 
        if { [scan $parameter {%[^=]=%s} name value] == 2 } {
            switch -glob -- $name { 
                txtUserid { 
                    set userid $value     
                }   
                txtTok* { 
                    set deny_user 1
                }   
            }
        }
    }   

    if { [info exists deny_user] } { 
        log local0. "Denying user - Redirecting Userid $userid to https://www.test.com/Log_On_Error.htm"; 
        HTTP::redirect https://www.test.com/Log_On_Error.htm 
    } else { 
        HTTP::release 
    }
}