adding new contact (from iphone) does not work (new item isnt in iphone nor nc)

Describe the bug

contacts sync is in place for over 1 year through the various versions and worked 2 weeks back.
I add a new contact on iphone (17.4.1) with fist-name = David and last-name=Pipapo, not supplying additional info.
Press save. The contact is not in the iphone.And not in NC either.
Adding same in NC (web access with FFox Browser on Mac) in the contacts app works well.

Steps to reproduce

1. Go to iphone contacs app. in the list "all XXX-contacts" (the default in the iphone)
2. Click on '+'
3. Scroll down to 'Vorname' (first-name), enter David
4. Next field "Nachname" (last-name), enter Pipapo
5. press top right "Fertig" (not sure how that button is called in engl., maybe "add", not sure)

Expected behavior

Contact is added on iphone and in NC instance under default "contacts".

Actual behavior

not added, neither on iphone nor NC contacts anywhere

Contact version

5.5.3

Operating system

FreeBSD13.2-p11

PHP engine version

PHP 8.1

Web server

Apache (supported)

Database

MariaDB

Additional info

iphone 17.4.1
Webserver log:
212.XX.151.59 - - [02/Apr/2024:13:31:01 +0200] "PROPFIND /nc/remote.php/dav/principals/users/myuser/ HTTP/2.0" 207 200 469 TLSv1.3 TLS_AES_256_GCM_SHA384 "-" "iOS/17.4.1 (21E236) dataaccessd/1.0"
212.XX.151.59 - - [02/Apr/2024:13:31:02 +0200] "PROPFIND /nc/remote.php/dav/addressbooks/users/myuser/ HTTP/2.0" 401 200 557 TLSv1.3 TLS_AES_256_GCM_SHA384 "-" "iOS/17.4.1 (21E236) dataaccessd/1.0"
212.XX.151.59 - - [02/Apr/2024:13:31:02 +0200] "PROPFIND /nc/remote.php/dav/addressbooks/users/myuser/ HTTP/2.0" 207 200 635 TLSv1.3 TLS_AES_256_GCM_SHA384 "-" "iOS/17.4.1 (21E236) dataaccessd/1.0"
212.XX.151.59 - - [02/Apr/2024:13:31:02 +0200] "REPORT /nc/remote.php/dav/addressbooks/users/myuser/contacts/ HTTP/2.0" 401 200 557 TLSv1.3 TLS_AES_256_GCM_SHA384 "-" "iOS/17.4.1 (21E236) dataaccessd/1.0"
212.XX.151.59 - - [02/Apr/2024:13:31:03 +0200] "REPORT /nc/remote.php/dav/addressbooks/users/myuser/contacts/ HTTP/2.0" 207 200 274 TLSv1.3 TLS_AES_256_GCM_SHA384 "-" "iOS/17.4.1 (21E236) dataaccessd/1.0"
212.XX.151.59 - - [02/Apr/2024:13:31:03 +0200] "PROPFIND /nc/remote.php/dav/addressbooks/users/myuser/z-app-generated--contactsinteraction--recent/ HTTP/2.0" 401 200 557 TLSv1.3 TLS_AES_256_GCM_SHA384 "-" "iOS/17.4.1 (21E236) dataaccessd/1.0"
212.XX.151.59 - - [02/Apr/2024:13:31:03 +0200] "PROPFIND /nc/remote.php/dav/addressbooks/users/myuser/z-app-generated--contactsinteraction--recent/ HTTP/2.0" 207 200 307 TLSv1.3 TLS_AES_256_GCM_SHA384 "-" "iOS/17.4.1 (21E236) dataaccessd/1.0"