Unstable: system has not been booted with systemd as init system

During the boot I’m seeing this one:

<<< NixOS Stage 2 >>>

running activation script...
setting up /etc...
System has not been booted with systemd as init system (PID 1). Can't operate.
Failed to connect to bus: Host is down
starting systemd...

Welcome to NixOS 21.03 (Okapi)!

rather than that, everything runs fine.

I’m not really sure what caused this, after I installed NixOS and later everything was fine, no errors whatsoever.

Any hint how to fix this?
Thanks.