Unable to create account

Over 5 days now I tried several times to get self-hosted Bitwarden running. Always on Proxmox within virtual machines or lxc containers with docker. OS’s Debian, Ubuntu, CentOS and Arch. Once with Debian vm worked, but doesn’t keep items in the vault and I can’t sync it. All other attempts failed with network errors or server errors. Some failed with SQL errors after restarting the server.

The last attempt gives me error: “Open registration has been disabled by the system administrator.” while trying to create an account.

Documentation doesn’t say anything about this error.