TYPO3 Core

If you use the FE_SESSION_KEY the sessionid is written in $_COOKIE[$this->fe_user->name] (tslib_fe::initFEuser()). Later in the workflow the skript looks for the sessionid in $_SERVER['HTTP_COOKIE'] (t3lib_userauth::getCookie()). But the sessionid was stored in $_COOKIE.

Here is my fixed version of tslib_fe::initFEuser():
function initFEuser() {
$this->fe_user = t3lib_div::makeInstance('tslib_feUserAuth');

$this->fe_user->lockIP = $this->TYPO3_CONF_VARS['FE']['lockIP'];
  $this->fe_user->checkPid = $this->TYPO3_CONF_VARS['FE']['checkFeUserPid'];
  $this->fe_user->lifetime = intval($this->TYPO3_CONF_VARS['FE']['lifetime']);
  $this->fe_user->checkPid_value = $GLOBALS['TYPO3_DB']->cleanIntList(t3lib_div::_GP('pid'));  // List of pid's acceptable

// Check if a session is transferred:
  if (t3lib_div::_GP('FE_SESSION_KEY'))  {
    $fe_sParts = explode('-',t3lib_div::_GP('FE_SESSION_KEY'));
    if (!strcmp(md5($fe_sParts[0].'/'.$this->TYPO3_CONF_VARS['SYS']['encryptionKey']), $fe_sParts[1]))  {  // If the session key hash check is OK:

if (isset($_SERVER['HTTP_COOKIE'])) {

$lastCookie = null;
        $cookies = t3lib_div::trimExplode(';', $_SERVER['HTTP_COOKIE']);        
        foreach ($cookies as $cookie) {
          list ($name, $value) = t3lib_div::trimExplode('=', $cookie);
          if (strcmp(trim($name), $this->fe_user->name) == 0) {
            // Use the last one
            $lastCookie = $cookie;
          }
        }
        if($lastCookie) {
          $_SERVER['HTTP_COOKIE'] = str_replace($lastCookie ,$this->fe_user->name .'='. $fe_sParts[0], $_SERVER['HTTP_COOKIE']);
        } else {
          $_SERVER['HTTP_COOKIE'] .= ';' . $this->fe_user->name .'='. $fe_sParts[0];
        }
      }

$_COOKIE[$this->fe_user->name] = $fe_sParts[0];
      $this->fe_user->forceSetCookie = 1;
    }
  }
}