Hi! I hit this as well while running `guix system reconfigure' (unfortunately I didn't capture the stack trace).
Re-running `guix system reconfigure' right after it had failed worked... So my guess is that it's a stateful issue, having to do with grafts: if polkit is not in the store, then we build/download it and fail right after that, which is why it succeeds the second time. To be continued... Ludo'.
