PROPFIND not returning the properties I ask for

[tobixen]

I have access to a sabre server, when accessing it through the browser it says "SabreDAV 4.3.1 (c)2007-2022", and it also lists up lots of properties, both for /cal.php, /cal.php/principals/, etc, including the d:current-user-principal property.

I try to pass this request to the server (I edited it, added some extra white space for clarity):

PROPFIND /cal.php/
User-Agent: Mozilla/5.0
Content-Type: text/xml
Accept: text/xml, text/calendar
Depth: 0

<D:propfind xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav">
  <D:prop><D:current-user-principal/></D:prop>
</D:propfind>

And I do get this in return (I've edited it, added some extra white space for readability):

<?xml version="1.0"?>
<d:multistatus xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns" xmlns:cal="urn:ietf:params:xml:ns:caldav" xmlns:cs="http://calendarserver.org/ns/" xmlns:card="urn:ietf:params:xml:ns:carddav">
  <d:response>
    <d:href>/cal.php/</d:href>
    <d:propstat>
        <d:prop><d:resourcetype><d:collection/></d:resourcetype></d:prop> 
        <d:status>HTTP/1.1 200 OK</d:status>
    </d:propstat>
  </d:response>
</d:multistatus>

The current-user-principal property is not returned. Apparently the body is ignored. Whatever body I send, it returns the same. I can send invalid XML, or simply "Hello world!" and it makes no difference. The request is sent via the test code for the python caldav client library - the test code works towards a lot of other caldav servers, I've never seen this problem with any other servers. Any hints on how I can proceed debugging this?