[Haskell-cafe] Strange No instance error with cabal install

[Rune Harder Bak]

Sometimes when one of our developers (using Arch-linux) tries to cabal
install packages he gets
a "no instance for" error.
Code that compiles fine on my computer, and other computers I try it
on. Even some packages on hackage
(trying to install aeson-native for instance gives Data/Aeson/Types.hs:196:22:
    No instance for (NFData Object)
      arising from a use of `rnf'
    Possible fix: add an instance declaration for (NFData Object)
    In the expression: rnf o
    In an equation for `rnf': rnf (Object o) = rnf o
    In the instance declaration for `NFData Value')
I tried removing the .ghc and .cabal folder, even reinstalled ghc-7.04
and cabal-install (most recent), but the problem persisted (might not
have been a clean install though, I'm no Arch expert)
I know this is kind of vauge, and not in she shape of filling a bug
report, heck I can't even reproduce the error on my computer.
But perhaps somebody else has run in to this strange behavior before?

Best,
Rune