Here is the full log
"/homepages/11/d4298927297/htdocs/surveys/polls/vendor/yiisoft/yii/framework/web/CHttpRequest.php(1400)1388
1389 if (!empty($maskedUserToken) && is_string($maskedUserToken) && $cookies->contains($this->csrfTokenName))
1390 {
1391 $securityManager=Yii::app()->getSecurityManager();
1392 $maskedCookieToken=$cookies->itemAt($this->csrfTokenName)->value;
1393 $cookieToken=$securityManager->unmaskToken($maskedCookieToken);
1394 $userToken=$securityManager->unmaskToken($maskedUserToken);
1395 $valid=$cookieToken===$userToken;
1396 }
1397 else
1398 $valid = false;
1399 if (!$valid)
1400 throw new CHttpException(400,Yii::t('yii','The CSRF token could not be verified.'));
1401 }
1402 }
1403
1404
1405 /**
1406 * Returns the version of the HTTP protocol used by client.
1407 *
1408 * @return string the version of the HTTP protocol.
1409 * @since 1.1.16
1410 */
1411 public function getHttpVersion()
1412 {
Stack Trace#0+
  /homepages/11/d4298927297/htdocs/surveys/polls/vendor/yiisoft/yii/framework/base/CComponent.php(559): CHttpRequest->validateCsrfToken()#1+
  /homepages/11/d4298927297/htdocs/surveys/polls/vendor/yiisoft/yii/framework/base/CApplication.php(212): CComponent->raiseEvent()#2+
  /homepages/11/d4298927297/htdocs/surveys/polls/vendor/yiisoft/yii/framework/base/CApplication.php(183): CApplication->onBeginRequest()#3–
  /homepages/11/d4298927297/htdocs/surveys/polls/index.php(161): CApplication->run()156 require_once APPPATH . 'core/LSYii_Application' . EXT;
157
158 $config = require_once(APPPATH . 'config/internal' . EXT);
159
160 Yii::$enableIncludePath = false;
161 Yii::createApplication('LSYii_Application', $config)->run();
162
163 /* End of file index.php */
164 /* Location: ./index.php */"

Session issue in my opinion