On a clean installation we see the following error in the log file after clicking on "Magazin" or "Weiteres" in the documents tree. If display_errors is enabled the loading animation keeps rotating and the child elements will never show up.

pimcore.ERROR: RuntimeException: Failed to start the session because headers have already been sent by "/var/www/pimcore/vendor/symfony/symfony/src/Symfony/Component/HttpFoundation/Response.php" at line 1219. in /var/www/pimcore/vendor/symfony/symfony/src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php:147 Stack trace: #0 /var/www/pimcore/vendor/symfony/symfony/src/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php(320): Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->start() #1 /var/www/pimcore/vendor/symfony/symfony/src/Symfony/Component/HttpFoundation/Session/Session.php(244): Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage->getBag('ecommerceframew...') #2 /var/www/pimcore/vendor/pimcore/pimcore/bundles/EcommerceFrameworkBundle/SessionEnvironment.php(124): Symfony\Component\HttpFoundation\Session\Session->getBag('ecommerceframew...') #3 /var/www/pimcore/vendor/pimcore/pimcore/bundles/EcommerceFrameworkBundle/SessionEnvironment.php(62): Pimcore\Bundle\EcommerceFrameworkBundle\SessionEnvironment->getSessionBag() #4 /var/www/pimcore/vendor/pimcore/pimcore/bundles/EcommerceFrameworkBundle/Environment.php(300): Pimcore\Bundle\EcommerceFrameworkBundle\SessionEnvironment->load() #5 /var/www/pimcore/vendor/pimcore/pimcore/bundles/EcommerceFrameworkBundle/IndexService/IndexService.php(320): Pimcore\Bundle\EcommerceFrameworkBundle\Environment->getCurrentAssortmentTenant() #6 /var/www/pimcore/vendor/pimcore/pimcore/bundles/EcommerceFrameworkBundle/IndexService/IndexService.php(198): Pimcore\Bundle\EcommerceFrameworkBundle\IndexService\IndexService->resolveTenantWorker(NULL) #7 /var/www/pimcore/vendor/pimcore/pimcore/bundles/EcommerceFrameworkBundle/CoreExtensions/ClassDefinition/IndexFieldSelectionCombo.php(46): Pimcore\Bundle\EcommerceFrameworkBundle\IndexService\IndexService->getIndexAttributes(true) #8 /var/www/pimcore/vendor/pimcore/pimcore/bundles/EcommerceFrameworkBundle/CoreExtensions/ClassDefinition/IndexFieldSelectionCombo.php(37): Pimcore\Bundle\EcommerceFrameworkBundle\CoreExtensions\ClassDefinition\IndexFieldSelectionCombo->buildOptions() #9 /var/www/pimcore/vendor/pimcore/pimcore/models/DataObject/ClassDefinition/Helper/VarExport.php(28): Pimcore\Bundle\EcommerceFrameworkBundle\CoreExtensions\ClassDefinition\IndexFieldSelectionCombo->__construct() #10 /var/www/pimcore/var/classes/fieldcollections/OrderByFields.php(61): Pimcore\Model\DataObject\ClassDefinition\Data::__set_state(Array) #11 /var/www/pimcore/vendor/pimcore/pimcore/models/DataObject/Fieldcollection/Definition.php(268): include('/var/www/pimcor...') #12 /var/www/pimcore/vendor/pimcore/pimcore/models/DataObject/ClassDefinition/Data/Fieldcollections.php(387): Pimcore\Model\DataObject\Fieldcollection\Definition::getByKey('OrderByFields') #13 /var/www/pimcore/vendor/pimcore/pimcore/models/DataObject/ClassDefinition/Data.php(335): Pimcore\Model\DataObject\ClassDefinition\Data\Fieldcollections->setAllowedTypes(Array) #14 /var/www/pimcore/vendor/pimcore/pimcore/models/DataObject/ClassDefinition/Helper/VarExport.php(29): Pimcore\Model\DataObject\ClassDefinition\Data->setValues(Array) #15 /var/www/pimcore/var/classes/definition_FilterDefinition.php(242): Pimcore\Model\DataObject\ClassDefinition\Data::__set_state(Array) #16 /var/www/pimcore/vendor/pimcore/pimcore/models/DataObject/ClassDefinition.php(200): include('/var/www/pimcor...') #17 /var/www/pimcore/vendor/pimcore/pimcore/models/DataObject/Concrete.php(448): Pimcore\Model\DataObject\ClassDefinition::getById('EF_FD') #18 /var/www/pimcore/vendor/pimcore/pimcore/models/DataObject/Concrete/Dao.php(147): Pimcore\Model\DataObject\Concrete->getClass() #19 /var/www/pimcore/vendor/pimcore/pimcore/models/DataObject/Concrete/Dao.php(57): Pimcore\Model\DataObject\Concrete\Dao->getData() #20 /var/www/pimcore/vendor/pimcore/pimcore/models/DataObject/AbstractObject.php(303): Pimcore\Model\DataObject\Concrete\Dao->getById(696) #21 /var/www/pimcore/vendor/pimcore/pimcore/models/Element/Service.php(443): Pimcore\Model\DataObject\AbstractObject::getById(696, false) #22 /var/www/pimcore/vendor/pimcore/pimcore/models/Document/Tag/Relation.php(175): Pimcore\Model\Element\Service::getElementById('object', 696) #23 /var/www/pimcore/vendor/pimcore/pimcore/models/Document/Tag/Relation.php(144): Pimcore\Model\Document\Tag\Relation->setElement() #24 /var/www/pimcore/vendor/pimcore/pimcore/models/Document/PageSnippet/Dao.php(54): Pimcore\Model\Document\Tag\Relation->setDataFromResource(Array) #25 /var/www/pimcore/vendor/pimcore/pimcore/models/Document/PageSnippet.php(507): Pimcore\Model\Document\PageSnippet\Dao->getElements() #26 /var/www/pimcore/vendor/pimcore/pimcore/models/Document/PageSnippet.php(209): Pimcore\Model\Document\PageSnippet->getElements() #27 /var/www/pimcore/vendor/pimcore/pimcore/models/Document/Link.php(119): Pimcore\Model\Document\PageSnippet->getCacheTags(Array) #28 /var/www/pimcore/vendor/pimcore/pimcore/lib/Cache/Core/CoreHandler.php(482): Pimcore\Model\Document\Link->getCacheTags(Array) #29 /var/www/pimcore/vendor/pimcore/pimcore/lib/Cache/Core/CoreHandler.php(847): Pimcore\Cache\Core\CoreHandler->prepareCacheTags(Object(Pimcore\Cache\Pool\CacheItem), Object(Pimcore\Model\Document\Link), Array) #30 /var/www/pimcore/vendor/pimcore/pimcore/lib/Cache/Core/CoreHandler.php(902): Pimcore\Cache\Core\CoreHandler->writeSaveQueue() #31 /var/www/pimcore/vendor/pimcore/pimcore/lib/Cache.php(202): Pimcore\Cache\Core\CoreHandler->shutdown(false) #32 /var/www/pimcore/vendor/pimcore/pimcore/lib/Pimcore.php(273): Pimcore\Cache::shutdown() #33 /var/www/pimcore/vendor/pimcore/pimcore/lib/Kernel.php(202): Pimcore::shutdown() #34 [internal function]: Pimcore\Kernel->Pimcore{closure}() #35 {main} [] []

After debugging it came out that the error occurs in the shutdown procedure when Pimcore tries to save the cache queue. Then it tries to determine the current tenant from the current session which has already been closed at that time.