I solved the issue by creating two folders and change ownership manually.
sudo mkdir /nix/var/nix/profiles/per-user/ubuntu
sudo mkdir /nix/var/nix/gcroots/per-user/ubuntu
sudo chown ubuntu:root /nix/var/nix/profiles/per-user/ubuntu
sudo chown ubuntu:root /nix/var/nix/gcroots/per-user/ubuntu
It feels like the current multi-user installation script does not create all necessary folders.
Thank you again for all the info.