I created a fresh NC 30 installation with existing files on the server. Synced those with “occ files:scan”. All works perfect on Android and via Web-Browser. On Windows I synced with Virtual File Support, also those synced and all my files are visible. I can double-click files and they download automatically and view them.
So far so fine. When I enable “Always keep on this device” on the root folder, it starts syncing but errors on various files - I force to resync all and it erros on other files.
Nextcloud Hub 9 (30.0.0)
Nextcloud desktop client 3.14.0
Thank you for your suggestions and support!