Describe with more detaild, On what step you going to error? Are you completedly install Siena package? Can you get index page if site? Can you navigate through site content when not logged in? Do you get error only after trying to login?
What king of webserver you are used and php version? (printscreen you phpinfo() screen )
(first of all) Clear file cache from datas/cache folder (only files should be deleted, not folder structure).
(for apache server) check .htaccess file for «mod_rewtite» enabled or disables (`RewriteEngine` parameter), Check base (RewriteBase "/" paramenter).