I have found a workaround for Kontact / Zimbra connectivity via GroupDAV. I mount the contact or calendar resource on Zimbra as a davfs2 filesystem, and then treat it like a regular directory containing vCard or .vcf files.
The davfs2 package has to be installed. I added entries like this to /etc/fstab
This allows the GroupDAV resources to be mounted by the corresponding users ,and gives them read-write permissions. Mountpoints for contactsDAV and calendarDAV are created in the user home directory. There are a couple of davfs2 config files. There are globla versions in /etc/davfs2, but as these are individual user mounts it makes more sense to use the per-user versions in ~/.davfs2. In ~/.davfs2/davfs2.conf make sure the line
http://localhost.localdomain/dav/test/Contacts /home/andrew/contactsDAV davfs user,rw,noauto 0 0
http://localhost.localdomain/dav/test/Calendar /home/andrew/calendarDAV davfs user,rw,noauto 0 0
is present and not commented out - without this I was unable to write back to GroupDAV. In ~/.davfs2/secrets you can store the username & p/w for the GroupDAV resource; this allows mounts to be scripted without requiring input of credentials. As this is in the user home directory privacy is maintained even though the file is plaintext.
http://localhost.localdomain/dav/test/Contacts test testtest
http://localhost.localdomain/dav/test/Calendar test testtest
Create an address book of type "Personal Contacts" and use the local mountpoint as the directory name. This method has the slight disadvantage that the "lost+found" folder appears as a sub-folder of the Personal Contacts folder. If instead you select "KDE Traditional Addressbook", then "Folder" I found that each edit in Kontact produced a new version of the contact in Zimbra (and another identically-named version of the file in the davfs2 mount), although Kontact only displays the current version.
I added a calendar of type "Calendar in Local Directory" and used the local mountpoint as the directory. NB Don't use "Local File" as Zimbra GroupDAV keeps each entry in a separate file (same goes for contacts).
For both calendar and contacts I was able to upload entries created locally in Kontact, as well as download from Zimbra. Entries are cached when Zimbra is offline, provided the davfs2 filesystem remains mounted.
Hat-tip to folks at Memotoo (even though they are French... ;-) ) for pointing me in this direction:-https://www.memotoo.com/forum/messag...idmessage=2313. It is a little bit cumbersome, but while native Kontact / Akonadi support seems flaky at best, if it actually exists at all, it will do for me.