IRULES - LB::reselect

OK, so, you may want to always specify the fqdn as lowercase and also lower what your matching against:

if { [string tolower [HTTP::host]] equals $fqdn }

I'd also reorder your logic as it's possible you'll mark the newly selected node as down:

when HTTP_RESPONSE {
  if { ([HTTP::status] == 500) or ([HTTP::status] == 404) or ([HTTP::status] == 503) } {
    if { [active_members $poolname] > $NbNode } {
      LB::down
      LB::reselect pool $poolname
    }
    else {
      LB::reselect pool $poolname
    }
  }
}

I'm pretty sure you could shorten the HTTP::status lookup too but I'm too tired to try.

Also not you don't have any spaces between your if's and the opening brace {.