using an old system (2019-05-30)
nix --version
nix (Nix) 2.2.2
sudo echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6
sudo nixos-rebuild switch --upgrade
[...]
updating GRUB 2 menu...
Warning: the new NixOS configuration has an ‘init’ that is
incompatible with the current configuration. The new configuration
won't take effect until you reboot the system.
warning: error(s) occurred while switching to the new configuration
This took a while, but was a success. So the bug was introduced after 2019-05-30?
Edit: After the successful upgrade I booted the new system. There, the error is still present.