This post saved my day today. Unfortunately I also do now know how the entry in the hardware-configuration got there but I at least removing it got my system running again.
I would also gues it is a bug though. Maybe running nixos generate-config adds this entry from a running container instance which leads to the error when on boot nixos tries to mount a location that does not exist anymore?
Will investigate. This is the first time I experienced this error even though I have been using NixOS and docker for a while. However I only have today with this machine left and I am pretty happy that it is running again for now. Will keep you updated