http://10.147.42.36/icingaweb2/setup 

#0 /usr/share/icingaweb2/library/vendor/Zend/Form.php(2280): Icinga\Web\Form\Element\CsrfCounterMeasure->isValid('341152815|56d5c...', Array)
#1 /usr/share/php/Icinga/Web/Form.php(1276): Zend_Form->isValid(Array)
#2 /usr/share/php/Icinga/Web/Wizard.php(279): Icinga\Web\Form->isValid(Array)
#3 /usr/share/icingaweb2/modules/setup/application/controllers/IndexController.php(44): Icinga\Web\Wizard->handleRequest()
#4 /usr/share/icingaweb2/library/vendor/Zend/Controller/Action.php(507): Icinga\Module\Setup\Controllers\IndexController->indexAction()
#5 /usr/share/php/Icinga/Web/Controller/Dispatcher.php(76): Zend_Controller_Action->dispatch('indexAction')
#6 /usr/share/icingaweb2/library/vendor/Zend/Controller/Front.php(937): Icinga\Web\Controller\Dispatcher->dispatch(Object(Icinga\Web\Request), Object(Icinga\Web\Response))
#7 /usr/share/php/Icinga/Application/Web.php(389): Zend_Controller_Front->dispatch(Object(Icinga\Web\Request), Object(Icinga\Web\Response))
#8 /usr/share/php/Icinga/Application/webrouter.php(109): Icinga\Application\Web->dispatch()
#9 /usr/share/icingaweb2/public/index.php(4): require_once('/usr/share/php/...')
#10 {main}

SOLVED: i just had to restart the Browser ;-)