Core

In my case the problem is that 'document.domain' in the iFrames is empty (not in the parent frame). This makes the browser fail.

In PreviewController->indexAction the "backendDomain" is evaluated like this:

$this->view->assign('backendDomain', GeneralUtility::getIndpEnv('TYPO3_HOST_ONLY'));

In TypoScriptFrontendControllerHook->hook_eofe it is done like this:

'backendDomain.' => array('value' => $GLOBALS['BE_USER']->getSessionData('workspaces.backend_domain'))

In my case, the backend_domain from the session data is empty. When I change it to

'backendDomain.' => array('value' => \TYPO3\CMS\Core\Utility\GeneralUtility::getIndpEnv('TYPO3_HOST_ONLY'))

it works. I don't understand why in the latter case sessionData is used and in the first case the environment variables.