It's 9.5 Premium.
Adding a questionmark to the end of the image url causes the image not beeing displayed in the docx.
Using a RewriteRule ^image/(.*)$ image.php?id=$1 to insert my image by ...
$docx = new CreateDocx();
$html = '<img src="https://www.mydomain.com/image/123">';
$docx->embedHTML($html);
... is also working.
That's confusing...