20.03 -> 20.09 fail with Permission denied

Not sure how to deal with this?
(I tried to ask this question on IRC, but was not able to format it correctly: hence trying again here).

nixos-rebuild switch --upgrade
unpacking channels...
building Nix...
building the system configuration...
these derivations will be built:
building '/nix/store/qhmv0fr973fy025mz9ybk4x43b8g1bdf-etc.drv'...
mkdir: cannot create directory '/nix/store/bsi9602zg1fk5m819rb03rw7z05pj3af-etc/etc/dconf/db/local.d': Permission denied
builder for '/nix/store/qhmv0fr973fy025mz9ybk4x43b8g1bdf-etc.drv' failed with exit code 1
cannot build derivation '/nix/store/ibpf1zkbx7nxq5hqk9cfb3di1ypjclb7-nixos-system-pcdell-20.09.2076.ffb3aab257e.drv': 1 dependencies couldn't be built
error: build of '/nix/store/ibpf1zkbx7nxq5hqk9cfb3di1ypjclb7-nixos-system-pcdell-20.09.2076.ffb3aab257e.drv' failed

Do you run the rebuild as root or a regular user?

As root.

Anyway. I no more have this computer at hand, but would like to understand what could have gone wrong?