Given URL is not a homeset URL

On a client's machine running Kerio Connect 10.x:

debug: Given URL is not a homeset URL
debug: ====================
debug: PROPFIND https://irsmail-int.xxx.de/ical/ksh@xxx.de/Calendar/
debug: {'User-Agent': 'vdirsyncer/0.19.2', 'Content-Type': 'application/xml; charset=UTF-8', 'Depth': '0'}
debug: b'\n        <propfind xmlns="DAV:">\n            <prop>\n                <current-user-principal />\n            </prop>\n        </propfind>\n        '
debug: Sending request...
debug: 207
debug: <CIMultiDictProxy('Connection': 'Close', 'Content-Encoding': 'deflate', 'Content-Type': 'text/xml; charset="utf-8"', 'Date': 'Sat, 14 Feb 2026 12:47:30 GMT', 'DAV': '1, access-control, calendar-access, calendar-schedule, calendar-auto-schedule, calendarserver-principal-property-search, calendarserver-sharing, calendarserver-sharing-no-scheduling, addressbook, calendar-default-alarms', 'MS-WebStorage': '08.01.10240', 'Server': 'WebServer', 'Transfer-Encoding': 'chunked', 'X-UA-Compatible': 'IE=edge')>
debug: <StreamReader 380 bytes eof>
debug: No current-user-principal returned, re-using URL https://irsmail-int.xxx.de/ical/ksh@xxx.de/Calendar/
debug: ====================
debug: PROPFIND https://irsmail-int.xxx.de/ical/ksh@xxx.de/Calendar/
debug: {'User-Agent': 'vdirsyncer/0.19.2', 'Content-Type': 'application/xml; charset=UTF-8', 'Depth': '0'}
debug: b'\n    <propfind xmlns="DAV:" xmlns:c="urn:ietf:params:xml:ns:caldav">\n        <prop>\n            <c:calendar-home-set />\n        </prop>\n    </propfind>\n    '
debug: Sending request...
debug: 207
debug: <CIMultiDictProxy('Connection': 'Close', 'Content-Encoding': 'deflate', 'Content-Type': 'text/xml; charset="utf-8"', 'Date': 'Sat, 14 Feb 2026 12:47:30 GMT', 'DAV': '1, access-control, calendar-access, calendar-schedule, calendar-auto-schedule, calendarserver-principal-property-search, calendarserver-sharing, calendarserver-sharing-no-scheduling, addressbook, calendar-default-alarms', 'MS-WebStorage': '08.01.10240', 'Server': 'WebServer', 'Transfer-Encoding': 'chunked', 'X-UA-Compatible': 'IE=edge')>
debug: <StreamReader 415 bytes eof>
error: The server returned something vdirsyncer doesn't understand. Error message: InvalidXMLResponse("Couldn't find home-set.")
error: While this is most likely a serverside problem, the vdirsyncer devs are generally interested in such bugs. Please report it in the issue tracker at https://github.com/pimutils/vdirsyncer/issues

curl -X PROPFIND https://irsmail-int.xxx.de/ical/ksh@xxx.de/Calendar/ 
<?xml version="1.0" encoding="UTF-8"?><a:multistatus xmlns:a="DAV:" xmlns:d="http://schemas.microsoft.com/exchange/" xmlns:e="http://schemas.microsoft.com/mapi/proptag/" xmlns:f="http://schemas.microsoft.com/repl/
" xmlns:g="urn:schemas:httpmail:" xmlns:c="urn:uuid:c2f41010-65b3-11d1-a29f-00aa00c14882/" xmlns:b="xml:"><a:response><a:href>https://irsmail-int.xxx.de/ical/xxx.de/ksh/Calendar/</a:href><a:props
tat><a:status>HTTP/1.1 200 OK</a:status><a:prop><a:childcount c:dt="int">2163</a:childcount><a:contentclass>urn:content-classes:calendarfolder</a:contentclass><a:displayname>Calendar</a:displayname><a:getetag>1dc6
e....35c00000000</a:getetag><a:hassubs c:dt="boolean">0</a:hassubs><a:href>https://irsmail-int.xxx.de/ical/xxx.de/ksh/Calendar/</a:href><a:id>1d...
35c</a:id><a:iscollection c:dt="boolean">1</a:iscollection><a:isfolder c:dt="boolean">1</a:isfolder><a:ishidden c:dt="boolean">0</a:ishidden><a:href>https://irsmail-int.xxx.de/ical/xxx.de/ksh/</a
:href><a:resourcetype><a:collection/></a:resourcetype><a:uid>1dc6e83c...</a:uid><d:foldersize c:dt="int">20559037</d:foldersize><d:outlookfolderclass>IPF.Appointment</d:outlookfolderclass>
<d:permanenturl>1dc6e83c...</d:permanenturl><e:x0ff40003 c:dt="int">2</e:x0ff40003><e:x360a000b c:dt="boolean">0</e:x360a000b><e:x3613001f>IPF.Appointment</e:x3613001f><e:x36e41102 c:dt="m
v.bin.base64"><b:v/><b:v>AAAAAALfudSc85NIrXcdrkydBtcHAO9xA46wz09Ci0YncpUYLY...

curl https://irsmail-int.xxx.de/ical/ksh@xxx.de/Calendar
BEGIN:VCALENDAR
PRODID:-//Kerio Technologies//Kerio Connect//EN
METHOD: PUBLISH
VERSION:2.0
...

Start by reproducing the two PROPFIND requests and comparing vdirsyncer's response parsing with the Kerio Connect XML shown in the report. Trace the path that raises InvalidXMLResponse("Couldn't find home-set"); done means the response is either handled correctly or the server incompatibility is clearly documented.