Well, you're using ghc-6.12 ... :-)

The most recent version of pandoc that Hackage claims to have built with ghc 6.12 looks to be 1.6. Rolling back that far eliminates the json dependency entirely, so I think it would solve your issue. Or you could use the Pandoc in Debian stable, which appears to be 1.5.1.

A more recent ghc would probably also work, of course, but I imagine you're trying the Debian stable version for a reason.