TYPO3 Core

The main issue is the following method of class t3lib_FlashMessageQueue:

protected static function getFlashMessagesFromSession() {
    $flashMessages = $GLOBALS['BE_USER']->getSessionData('core.template.flashMessages');
    return is_array($flashMessages) ? $flashMessages : array();
}

This is why a BE-user has to be logged-in in the Extbase (flashmessage) context. The code logic is replaced by

protected static function getUserByContext() {
    return TYPO3_MODE === 'BE' ? $GLOBALS['BE_USER'] : $GLOBALS['TSFE']->fe_user;
}

in TYPO3 4.5.3, which looks appropriate to me.

Since the public API of class t3lib_FlashMessageQueue is the same for both branches, it might be possible to just use the 4.5 variant in 4.4. This is what I do as a workaround at the moment.

Best regards
Joachim

Some further information: Extbase 1.2 seems to work perfectly fine with TYPO3 4.4. But I have to use Extbase version 1.3.0 with TYPO3 4.4.