Altough there is an entry in the systemd unit file EnvironmentFile=/mnt/dietpi_userdata/vaultwarden/vaultwarden.env but I think this had to be symlinked to /mnt/dietpi_userdata/vaulwarden/.env.
I did not try with a symlink, I just renamed vaultwarden.env to .env and the info in journalctl went away.
In this file are already made some configs (like the data directory is set to /mnt/dietpi_userdata/vaultwarden), so it looks like it’s working?
In the systemd unit is also set a path to this file. So maybe it’s used because of the systemd unit? But if I would start vaultvarden manually, without systemd, the file would be ignored due to “false” naming?