@santabroo I did once have all my Calibre Web books stored on a Hetzner Cloud Volume (different from their storage boxes) and have that Mounted as Cloudron Volume.
I was rsync-ing my local Calibre Library up to that Hetzner Cloud Volume. I'm not sure if I ever tried upload via the Calibre Web interface too, but if I did I can't remember what happened.
I think it very likely would work though if you moved your Calibre Web app Data Directory to a Heztner Cloud Volume but of course this would only be possible if your Cloudron is running on a Hetzner Cloud VPS.