To the solution:
The problem seems to lie in the fact that Apple MacOS X does not get the assignment of the /.well-known parameter correctly. Accordingly, we have (at first) extended the .htaccess file with an addition.
1. Change htaccess file
Redirect 301 /.well-known/carddav /owncloud/remote.php/dav Redirect 301 /.well-known/caldav/owncloud/remote.php/dav
These additions must be in the .htaccess file.
2. Customize certificates
The adaptation of .htaccess has then (at least) led to some computers to success. However, just under Mojave this did not lead to the final solution. After we could no longer downgrade our computers, there was no choice but to search further. Then we scoured the endless log files of our OwnCloud server. This revealed the next problem: the other Macs simply did not trust our Synology server. Thus, the solution was very simple. It simply became on a computer, in which the aforementioned modification of the. Htaccess file has already led to success, exported the certificate of the server and at the computer, where it does not work, recorded. After setting the appropriate trust settings, as shown in the screenshot below, the exchange worked fine.