No Rendering in Flow 7.3

[regioadmin]

Hi,
I can't create pdfs under neos flow 7.3 - neither as download, save or inline.
I always get a blank page in the browser - no error message in the logs. It uses mpdf library for generation.
In previous versions it worked like a charm. What's wrong?

my composer.json

"require": {
        "neos/flow": { "~7.3.0",
        "neos/welcome": "~7.3.0",
        "neos/behat":"~7.3.0",
        "happy/coding":"@dev",
        "famelo/pdf":"^4.2"

Action

/**
   * @return void
   */
    public function pdfAction() {
		$document = new \Famelo\PDF\Document('Happy.Coding:Test');
		$document->assign('someVariable', 'foobar');
		$document->send();
		$document->download('Test' . date('d.m.Y') . '.pdf');
		$document->save('' . date('d.m.Y') . '.pdf');
	}

Template Test.html in /Happy.Coding/Resources/Private/Documents/

{namespace fm=Famelo\PDF\ViewHelpers}
<f:layout name="PDF"/>
<f:section name="Content">
	<p>HELLO WORLD</p>
</f:section>

[regioadmin]

help please!

[regioadmin]

/flow-base-distribution/Packages/Libraries/mpdf/mpdf/tmp/ was not writeable