InvalidXMLResponse("Couldn't find home-set.") with Apple/iCloud

• vdirsyncer version 0.16.8
• Python 2.7.18
• debian bullseye/testing; kernel release 5.10.0-5-amd64
• I have removed usernames, passwords, DSid, and pGUID from the pasted below. they are correct and work in Thunderbird/Lightning.
• Contacts sync, calendar does not.
• config file:

[general]
status_path = "~/.vdirsyncer/status/"
# CARDDAV
[pair josh_contacts]
a = "josh_contacts_local"
b = "josh_contacts_remote"
collections = ["from a", "from b"]
conflict_resolution = "b wins"
[storage josh_contacts_local]
type = "filesystem"
path = "~/.contacts/"
fileext = ".vcf"
[storage josh_contacts_remote]
type = "carddav"
url = "https://contacts.icloud.com"
username = "[removed]"
password = "[removed]"
# CALDAV
[pair josh_calendar]
a = "josh_calendar_local"
b = "josh_calendar_remote"
collections = ["from a", "from b"]
conflict_resolution = "b wins"
metadata = ["displayname", "color"]
[storage josh_calendar_local]
type = "filesystem"
path = "~/.calendars/"
fileext = ".ics"
[storage josh_calendar_remote]
type = "caldav"
url = "https://p48-caldav.icloud.com/[correct DSid here]/calendars/[correct pGUID here]"
username = "[removed]"
password = "[removed]"

• debug info:

debug: Sending request...
debug: 207
debug: {'Server': 'AppleHttpServer/4b0f4e40276cabbbae998f715d1fd1336e7478a2', 'Date': 'Wed, 31 Mar 2021 17:23:21 GMT', 'Content-Type': 'application/xml; charset=UTF-8', 'Transfer-Encoding': 'chunked', 'Connection': 'keep-alive', 'X-Apple-Jingle-Correlation-Key': 'YBEH6263NFFOFGZ7QYIBE2KMCU', 'apple-seq': '0', 'apple-tk': 'false', 'Apple-Originating-System': 'UnknownOriginatingSystem', 'X-Responding-Instance': 'caldavj:34801301:mr28p48ic-ztdg09161301:8501:2104B588:6024037a373c', 'X-Apple-API-Version': 'NONE', 'content-encoding': 'gzip', 'Strict-Transport-Security': 'max-age=31536000; includeSubDomains;', 'via': 'xrail:mr47p00ic-qujn01122301.me.com:8301:20D313:grp22,631194250daa17e24277dea86cf30319:c0b14ae6a05dcecf80b17b845c43e5e0:New York', 'X-Apple-Request-UUID': 'c0487f6b-db69-4ae2-9b3f-861012694c15', 'access-control-expose-headers': 'X-Apple-Request-UUID,Via', 'X-Apple-Edge-Response-Time': '210'}
debug: <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
debug: <multistatus xmlns="DAV:">
debug: 
debug:     
debug:     
debug:     
debug:         
debug:         <response xmlns="DAV:">
debug:             <href>/[DSid removed]/calendars/[pGUID removed]/</href>
debug:             
debug:                 <propstat>
debug:                     <prop>
debug:                         
debug:                             
debug:                             
debug:                         
debug:                     </prop>
debug:                     <status>HTTP/1.1 200 OK</status>
debug:                 </propstat>
debug:                 
debug:                     <propstat>
debug:                         <prop>
debug:                             
debug:                                 
debug:                                     <calendar-home-set xmlns="urn:ietf:params:xml:ns:caldav"/>
debug:                                 
debug:                             
debug:                         </prop>
debug:                         <status>HTTP/1.1 404 Not Found</status>
debug:                     </propstat>
debug:                 
debug:             
debug:         </response>
debug:         
debug:     
debug:     
debug:     
debug:     
debug: 
debug: 
debug: </multistatus>
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
error: 1 out of 2 tasks failed.

Start with the reported CalDAV configuration and debug XML response, reproducing the request against the provided iCloud calendar URL. Trace how the response handles the missing calendar-home-set property and compare the server's 404 response with the expected behavior. Done means the failure is understood and the issue can be resolved or clearly identified as an iCloud server-side incompatibility.