It seems you’re encountering issues with thumbnail previews in your Nextcloud installation. If you’re using a TurnKey Linux template inside a Proxmox LXC container, this setup can introduce various limitations — and thumbnail generation problems might be one of them.
We discussed the risks of using TurnKey templates for Nextcloud in more detail here:
Is there limitations to installing Nextcloud via CT template on Proxmox?
These templates often come with outdated or misconfigured components (like PHP or image processing libraries), which can break or limit core Nextcloud features — including thumbnail generation for image files.
Recommended setup: A Proxmox VM running Ubuntu Server 24.04 with Nextcloud All-in-One (AIO) installed via Docker offers a much more stable and compatible environment.
In this setup, thumbnail generation for image files works perfectly out of the box — no tweaks or manual fixes needed.
This approach provides full compatibility with all Nextcloud features, reliable updates, and long-term maintainability, without the hidden pitfalls of container templates.