Thanks for your ideas. Can you guide me to the "Nextcloud External Storage setting"-page where I can enable "show hidden files"?
I disabled the "Windows compatibility" Setting and enabled the hidden files in the gear-setting (left down in the files-web-page), but didn't find any setting especially for hidden files for webdav (the web-page-files shouldn't make a change for the web-dav-access, I suppose.
I changed the Joplin-path in the android app from domain.de/remote.php/webdav/Joplin to domain.de/remote.php/dav/files/username/Joplin - that led to the error message "File with name /Joplin/.resource... could not be located). I found that as "webdav-in that left-down-gear-settings...
Both path settings show "success" after the sync-check-button...
thanks for further help!
Christian