Thanks for the input.

The URL of my webdav-server contains the word zotero at the end, because I created the account with the name zotero. Within this account there is a subdir zotero, which should contain my data. That was the way it has been before.

At the moment I have no data in the webdav-server space, so I cannot lose data there. I had the same problem some years ago. I cannot reproduce the solution now.

The test of the webdav-server address says ok, thereafter a quite short sync time which in no way can sync my data and then syncing ends without error message. But there is nothing on the server.

No solution so far. What I have tried without success:

In my Nextcloud server I had activated server side encryption. I thought this might be a problem and have reinstalled the server without encryption.

Webdav-URL of the account with name "zotero" has the word ...../zotero at the end. Whenever I insert this address into the server address field, the "zotero" is knocked away. The same after manually inserting it. I created manually a subdir "zotero" in the account. That did not change anything. Server verification worked but not syncing worked.

Then I created a webdavserver-account with a different name "refman", containing /refman at the end of the URL. This was not knocked away after inserting. I created a subdir /zotero, server verification is stated to have worked. There should be many hours of synchronisation between my desktop and the empty webdav-server in the internet, but a few seconds after starting syncronisation it ends without any error message and no new files on the server side.

Any idea where I am going into the wrong direction?
What could I reasonably reset when I want to get all my files to the server, which is empty at the moment?
Server encryption is probably not possible to work with Zotero attachments?

@ dstillman
your comment came at the same moment as I sent mine.
The reason why I try to have the /zotero subdir is that I don't want to have the home dir of this account clogged with the many subdirs of Zotero attachments. That was the way I had it before with the old installation, where the URL did not end with ...zotero.
I just tried again with the ..../refman URL account, resetted file sync history, started sync, it ended after about a second as before.....

To be clear, Zotero will store its files in a folder in called "zotero" in your WebDAV account. You don't need to manually tell it to do that. What is the purpose of you wanting to have two nested "zotero" folders?

@bwiernik - this is a misunderstanding. I do not WANT to have two nested folders. It just results from what the system gives me. I have space at a commercial provider which supports Nextcloud ( www.nextcloud.com ) . I have installed a Nextcloud instance here and as admin I can create accounts as I need. I have created an account for my Zotero files with the account name and login "zotero". In the home directory of this account there is no subdir after creating it. From within this home directory the owner receives the URL of the home dir of this account from the server - it always has the name of the account at the end, in this case "zotero". A separate new dir /zotero created in this account has therefore an URL with two "zotero"s at the end of the address. That was the reason why I created an account named "refman" where the extra "zotero" was not kicked out. But that did not work either.

@dstillman - I tried with /zotero/zotero - both were stripped, but this is the minor problem, since I can use the account refman or whatever I create. So I tried with the account refman. .....
ok, file sync ist successfully set up
File sync history resetted
"The File Sync History for "My Library" has been cleared.
Ok, ok.
Clicking on the green sync circle
Ends after a few seconds, when placing curson on it
Sync with Zotero.org
Last Sync 1 min ago
Again resetting of file sync history
Restart with logging
Syncing - same as usual, ends after a few seconds.

D1434734418 is the Debug ID

Maybe a problem with Zotero not having rights to write files there??

@dstillman

Thanks for the very probable problem solution!

I know this error message, it has been present also in my last installation, too, but I didn't think it inhibited syncing - I just thought that these notes will not be synced, that's all. But maybe I did not follow the progress of syncing. Syncing is important for my two other Zotero installations in my laptop and the desktop in my condo, where I haven't been during the time which we used to call "winter" earlier. I used the laptop quite a lot and think that syncing worked but I never really checked.

I have deleted the notes with the images in them and the syncing circle is rotating already for quite a while, though no data have appeared in the /zotero directory yet. Will let it run over night and sleep with fingers crossed....

Could the program be changed so that it just omits these notes with images and does not stop syncing completely?

Thanks again for all input to solve my problem.

Files appearing in the webdav-server space!!!! Great!!!
Now problem seems to be really solved finally.