Managed server @ Hetzner means, managed by someone else but you. No root access.
Did you install the nextcloud image that Hetzner offers to install with ConsoleH?
Maybe Hetzner wants to restrict the posibility to change the logo. I realy don’t know.
What I know is, that it is possible to restrict that posibility, but as I said, managed by someone else, means you will have to ask Hetzner or maybe someone here who uses managed server @ Hetzner can help.