Sorry but I wasn't at home . Your patch work for me too. In chwala composer.json has: "sabre/dav" : "~2.1.11" I'm not specially attached with this version :)

Since there is only one server active using owncloud i keep the patch as it is and it works.
I changed the line as follows, you can see what I log :
// $is_dir = in_array('{DAV:}collection', $props['{DAV:}resourcetype']->ResourceType);

$is_dir = isset( $props['{DAV:}resourcetype']);

So maybe we can just change in:

$size = $response['headers']['content-length'][0];

I didn't logged the $response, good point. The server is returning the header but I think the format is different.