Nextcloud had some problems with the desktop client of the 3.4 series. Currently, all known issues are fixed with version 3.4.2 – except for one issue that only occurs with the community build of Nextcloud 23.
According to Nextcloud, this error does not occur with the Enterprise version. As a Nextcloud partner, we will be happy to assist you with the installation and configuration of the Enterprise builds.
Until then, it is strongly recommended to upgrade to the desktop client 3.4.2 to avoid possible data loss. A fix for the Nextcloud 23 issue is expected this week or early next week.
Anyone affected by data loss should take a look at the mtime correction tool kit. It searches for problematic files in the file system of the Nextcloud server, lists them and can repair them in the file system as well as in the database.