After digging into the code a little bit deeper, I now think I am on to something. I think that this issue has nothing to do with UC, rather with the fact how Drupal handels user sessions. Especially session handling when a anonymous user loggs in (and viceversa). I think that this thread (http://drupal.org/node/280934) might have something to do with it. Commenting out 'session_regenerate_id () form the following code actually overcomes the issue:

-- from session.inc in the Drupal core include files ---

/**
* Called when an anonymous user becomes authenticated or vice-versa.
*/
function sess_regenerate() {
$old_session_id = session_id();

// We code around http://bugs.php.net/bug.php?id=32802 by destroying
// the session cookie by setting expiration in the past (a negative
// value). This issue only arises in PHP versions before 4.4.0,
// regardless of the Drupal configuration.
// TODO: remove this when we require at least PHP 4.4.0
if (isset($_COOKIE[session_name()])) {
//setcookie(session_name(), '', time() - 42000, '/');
}

//session_regenerate_id();

//db_query("UPDATE {sessions} SET sid = '%s' WHERE sid = '%s'", session_id(), $old_session_id);
}

But, I am not quite sure yet how this affects other modules or Drupal core functions.