Parsing an .odt content.xml with PHP results in an empty object [closed]

I’m trying to parse a libre office writer content.xml file on my server with php simplexml_load_file.

$xml = simplexml_load_file("https://ff-webdesigner.com/erechnung/content.xml");
print_r($xml);

But it always results in an empty object SimpleXMLElement Object ( ). Why?
Checked with file_get_content, setting namespaces and much more. All in vain.
With other xml files it’s working flawlessly…i checked for xml validity and also fixed all minor warnings.