guixbuild does not exist???

[Konrad Hinsen]

Hi Guixers,

Today guix greeted me with a strange error message when I tried to 
install a package:

  guix package: error: build failed: the group `guixbuild' specified in 
`build-users-group' does not exist

I call this strange because
 1) The group exists, as do the users guixbuilder01 to guixbuilder10.
 2) Guix has been working for quite a while on this machine, and 
nothing spectacular happened since the last use.
 3) I cannot find any trace of this error message in the Guix source code.
 4) The guix daemon is running without any sign of trouble.

Any idea of what could be going wrong here?

Konrad.