Having experimented a bit, I found that unordered-containers-0.2.2.1 shipped with Ubuntu 13.04 is incompatible with ghc-7.6.2 (also shipped by Ubuntu). When I install the latest unordered-containers from Hackage it works (but it conflicts later with other packages).
It seems that the Haskell environment shipped by Ubuntu is botched. Purging all libghc-* packages and reinstalling from Hackage instead, fixed everything.