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 It seems, that outdated calendar entries are not synced.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.
