This is how it works: if you're not logged in, you'll get a 503 if the site has used too much bandwidth recently. If you are logged in as a member, you'll get a 503 if the server load is too high. If you have a privileged or higher account you should never get 503s.
i checked the hash against an online calculator and i think my routine to calculate sha1 is ok (even if the sha1 hash on my cookie is different ???)
It should be the same as the hash in your cookie. You must be calculating it wrong. If you're getting this page it means you're not logged in because the hash is incorrect. If you're getting this it means you're logged in but the server is overloaded. Try again in a couple minutes.
I have the same problem with the IQDB update script. Maybe I should switch to using cookies instead.
Does the script use a privileged account? It shouldn't make a difference whether you pass your login credentials in a cookie or as URL parameters.