Updated to NC 12 today.
The error log on the Windows client (Version 2.3.1 (build8) has several entries like
22.05.2017 13:15:47, Mailstore/Data/MailStoreFileGroup.fdb, F:\NC,Das Element ist aufgrund vorheriger Fehler nicht synchronisiert: Error transferring https://www.domain.tld:555/remote.php/dav/uploads/kdj/1804273858/.file - server replied: Forbidden
The file in question is 322 MB in size.
I have already tried “Force sync now” but the situation does not change. I’m running Nextcloud on my own server, Ubuntu 17.10, php 7, nginx, PostgreSQL. My php.ini and www.conf is setup as described here: https://www.c-rieger.de/nextcloud-installation-guide/#c02