I'm not sure but I guess path_redirect should strip a leading slash of the redirect url. Something like:

  // Strip leading slashes.
  $form_state['values']['redirect'] = preg_replace('%^/+%','', $form_state['values']['redirect']);

What do you think? I could create a patch for this, but first I want some feedback.

May we also strip the base_url (before stripping the slash)? 

  global $base_url;

  // Strip leading base_url.
  $form_state['values']['redirect'] = preg_replace('%^' . preg_quote($base_url) . '%','', $form_state['values']['redirect']);