Thank you! Fixed the temp dir path in /etc/php/7.3/fpm/php.ini, rebooted the server, and now my Android and iOS clients synchronize as they should.
However, my Linux Mint laptop has problems: the 2.6.1git client seems to connect but nothing happens, the Activity window is blank.
Logging page (web interface) shows several lines like this:
[PHP] Error: fread(): file created in the system’s temporary directory at /var/www/nextcloud/3rdparty/icewind/streams/src/Wrapper.php#91
PUT /remote.php/webdav/Photos/2019/12/19-12-13%2023-06-36%200001.jpg
from xx.xx.xx.xx by oves at 2019-12-18T12:24:38+00:100:
Should I “reset” the syncing somehow?
Edit: uninstalled Nextcloud-client and removed manually all config and other related files I could find, and then reinstalled. Now it’s resyncing the 31GB of stuff and seems to work as it should.