[development] external authentication - not found error

[Jakub Suchy]

Hi,
i am developing small authentication module on 4.7 + patch allowing
authenticating without @domain

My module is very simple (code here is little bit condensed):
function auth602_help($section = '') { ... }

function auth602_info($field = 0) {
        $info['name'] = '602Office'; $info['protocol'] = 'XML-RPC';
        if ($field) { return $info[$field]; }
        else { return $info; }
}

function auth602_auth($username, $password, $server) {
  // authenticate
  if ($authenticated) {
   return 1;
  } else {
   return 0;
}

// end of module

Everything works, my "Default authentication server" is "xyz.cz", so
when user write "someuser", hook_auth() processes someuser at xyz.cz
But, after authenticating, user is logged in and Drupal throws error:

"Error -404 : not found"

I am trying to debug where this message come from (for 3 hours now :(.
Is there any way to look up module or function which throwed this error
or do anybody know why this error appears? Nothing else is wrong. Just
this message. User still authenticated.

Thanks,
Jakub Suchy
http://www.drupal.cz