Calendar update creates complete new calendar with same url

[spinne1000]

Hello,
I have a collection (Radicale running on a Debian Server)
root@server# pwd
/var/lib/radicale/collections/collection-root/testuser
root@server# ls -ldi 6579*
45884007 drwxr-x--- 3 radicale radicale 4096 Jul 25 14:36 65791233-cf20-7436-8baa-b13eddd5f14e
root@server# ls -li 6579*
45884070 -rw------- 1 radicale radicale 349 Jul 25 14:36 1c8f4e50-1981-62b8-cc61-2047c40538d2.ics
45884053 -rw------- 1 radicale radicale 284 Jul 25 14:32 c1043f76-4039-415f-9d94-941498130824.ics

This is the calendar I am working with. I have included it in thunderbird. If I change an event in this calendar, synching it an look at the inodes again:
root@server# ls -ldi 6579*
45884007 drwxr-x--- 3 radicale radicale 4096 Jul 25 14:40 65791233-cf20-7436-8baa-b13eddd5f14e
root@server# ls -li 6579*
45884124 -rw------- 1 radicale radicale 349 Jul 25 14:40 1c8f4e50-1981-62b8-cc61-2047c40538d2.ics
45884053 -rw------- 1 radicale radicale 284 Jul 25 14:32 c1043f76-4039-415f-9d94-941498130824.ics

Only the directory and the first event is changed, but the inods are the same. The calendar and the event is updated.

I want to do the same now with an c# WebDavClient. I have a CalDav-library which is not relevant. With the help of this library, I create the following string, which will be sent to the server:

Console.WriteLine("my serialized calendar: " + serializedCalendar);

(gives the following output)

BEGIN:VCALENDAR
PRODID:-//github.com/rianjs/ical.net//NONSGML ical.net 4.0//EN
VERSION:2.0
BEGIN:VEVENT
CREATED:20220712T113009Z
DTEND:20220726T043000
DTSTAMP:20220725T123205Z
DTSTART:20220726T033000
SEQUENCE:0
SUMMARY:Test5
UID:f8267df0-f3ff-4972-9999-cfc59fe3aa3d
END:VEVENT
BEGIN:VEVENT
CREATED:20220721T161715Z
DTEND:20220727T060000
DTSTAMP:20220725T123205Z
DTSTART:20220727T050000
SEQUENCE:0
SUMMARY:TestA
UID:0373f7c1-9778-46d1-9251-e278fc723b56
END:VEVENT
END:VCALENDAR

Console.WriteLine(CalendarUri);  // gives:

/testuser/65791233-cf20-7436-8baa-b13eddd5f14e/

 byte[] bytes = Encoding.UTF8.GetBytes(serializedCalendar);  
 MemoryStream stream = new MemoryStream(bytes);

 var responseCalendar = await WebDavClient.PutFile(CalendarUri, stream);
 Console.WriteLine("responseCalendar: " + JsonConvert.SerializeObject(responseCalendar, Formatting.Indented));

(gives the following)

responseCalendar: {
"StatusCode": 201,
"Description": "Created",
"IsSuccessful": true
}

And the same as on top of this posting:

root@server# ls -ldi 6579*
41418756 drwxr-x--- 3 radicale radicale 4096 Jul 25 14:50 65791233-cf20-7436-8baa-b13eddd5f14e
root@server# ls -li 6579*
41428972 -rw------- 1 radicale radicale 284 Jul 25 14:50 560219b6-c733-506d-580d-a6f0a7acbe78.ics
41428969 -rw------- 1 radicale radicale 284 Jul 25 14:50 e590b26c-16cd-a86f-aa4c-764839dee052.ics

The inode of the directory changed, what means it was created new. And so the events are completely new.

What have I to do, to get the calendar updated and not created new?