Sync of contacts between Nextcloud and Thunderbird doesn't work - but calendars do

Many years the sync between the calendars in my Thunderbird and Nextcloud are doing very well and they do today also.
But the contacts won’t sync, allthough I have used the link, which is available from Nextcloud web interface like:
https://mynextcloud.de/remote.php/dav/addressbooks/users/Clemens/Clemens Adressen/

There is no error message, when I do syncing manual from Thunderbird.

Somewhere in the web I have found the hint, that I should install the Thunderbird AddOn “CategoryManager” and “Provider for CalDAV & CardDAV” and “TbSync” also. In TbSync, I could set the URL to my Nextcloud Server and TbSync told me, that it could access successfully to the server.
But all the efforts with these AddOns did not bring success.

I have about 160 contacts in Thunderbird in 26 categories, but in Nextcloud, I only have 54 contacts in 13 categories. How can I do a sync, so that the additional adresses from Thunderbird will be synced completely with their categories with my Nextcloud?

What should I do generally, to get Thunderbird contacts synced with my contacts in Nextcloud?

I recommend to use the CardBook addon to sync your contacts with Thunderbird. Once the exsitng address books are synced you can move existing contacts from local address books to the Nextcloud address books.

Sorry to say, that my complete Nextcloud Server crashed yesterday. I took several backups from the data folder within Nextcloud installation as well as by export of calendars and cardbook in Thunderbird.
The I did a complete and clean new installation of Nextcloud. After setting up me as a user, I used the same username / passwords as in the previous installation. I hoped, that I could sync with my Thunderbird again. But this did not happen!

In the Web-UI of Nextcloud, logged in as the user, I can see in settings/security/ that the Thunderbird CardBook 75.5 Addon did connect successfully with Nextcloud. But there is no syncing neither from Thunderbird to Nextcloud nor the other direction.

Further there seemed to be no connection between Thunderbird calendar to Nextcloud.

I have checked very carefully that the links to calendar and to cardbook are correct!

The connection from my Android via DAVx5 to Nextcloud was successfully (DAVx5 was listed in the list in Nextcloud settings/security), but it did not sync the calendar. Instead it deleted all contacts / cardbook and also all tasks on the smartphone.

I tried for 6 hours but without success. Now I don’t have any clue, how to go on.

Sorry but I had to change the title of this theme, after I had to reinstall NC complete!

NC is latest 24.0.1. Thunderbird is latest = 91.9.1 its AddOn CardBook is latest = 75.5 and on Android the DAVx5 is 4.2.1-ose.
I have carefully checked the sync links for calendars:
https://my-nc-server.de/remote.php/dav/calendars/user-name/personal/
https://my-nc-server.de/remote.php/dav/calendars/user-name/tasks/
and for contacts:
https://my-nc-server.de/remote.php/dav/addressbooks/users/user-name/contacts

These are the results, when syncing with Thunderbird and the AddOn CardBook:
Calendars (personal) will not be synced.
Calendars (tasks) will be synced both, when tasks are added or deleted
Contacts: New contacts and groups will be synced with NC only, if they will be added. If I want to delete a contact or a group, this will not synced with NC.

These are the the results, when syncing with Android, DAVx5 and a contact app, a calendar app (etar) and tasks app:
Calendars (personal) will not be synced.
Calendars (tasks) will be synced both, when tasks are added or deleted.
Contacts and groups can be added and deleted.

It seems to be very unusual, that the calendar (personal) cannot be synced on both clients, but the calendar (tasks), which technically is based on the same principal, does fully syncing on Android and in part on Thunderbird.

I want to add: There are no errors in the Nextcloud logfile nor in the server logfile.

As before, I do not have any clue, how to get on now, to get rid of these bugs.

Update info: Actually only the contact / address book (in Thundebird) is causing problems!

When I did a fresh installation of Nextcloud server, I assumed, that it would be sufficient, to use the same credentials and calendar names etc… Then reconnect my clients with the NC server, in the believe, that then all tasks, contacts and calendars would be transferred automatically from the clients to the server.

Then I discovered, that this would not happen. While all the tasks had been transferred this way to NC server correctly from Thunderbird, all tasks on my Android had been deleted by sync at first and after that, they were synced again.
Tasks are synced well between the clients Thunderbird and Android and the NC server!

But no entry from the calendar had been transferred from Thunderbird or from Android to the NC server. And contrary to the task calendar, nothing was deleted by syncing. The only solution for me, is to export the calendar into an *.ics file and then import it in NC server. My assumption, calendar would not be synced, was wrong. It seems, that outdated calendar entries are not synced. New entries are syncing well. – I have tried it this morning.

Remaining trouble: the contacts / address book!
To start fresh, I have set up an new address book in Thunderbird and did connect this with the NC server. After that I copied all my contacts from the local address book to the new one in Thunderbird.
I controlled this process via the web access of NC. I could see, how not only the contacts but also their category / group was also copied and transferred to the NC server.

But when I deleted on of these contacts in Thunderbird, this did not sync with NC server.

When I restarted Thunderbird some hours later, all contacts of the synced address book were gone! But they were still there on NC server! And they were still synced with my android.
Lucky I have a copy of all these contacts in my local address book in Thunderbird. I copied all the contacts in that address book which is synced with NC server and did a sync in Thunderbird. On the NC server this had no effect.
Then I changed something in one contact on the NC server. This changing was not synced with Thunderbird.
I deleted a contact from Thunderbird. This was synced with NC. The I renewed the same contact in Thunderbird. This was not synced with NC.
When I closed Thunderbird and then started it new, all the contacts were gone!

Thus it seems, we have a problem in Thunderbird and not in NC.

BTW: I had a folder on my desktop PC, which was completely synced with all my files on NC. When I had setup the NC server fresh, this folder was automatically synced with NC. I assumed, that the difference (no files in NC and all files in the local folder) would be egalized, so that all the local files would be transferred to the NC server. But instead all the local files were deleted!
I was lucky to have local stored backups, so that I could restore all this stuff again.

I want to share my observations here, because the question, when and in which direction something is synced with NC is of some importance.