So I printed off the requirements for pandoc on a empty ghc-7.6.2 install you can find it at:
I do not see any odd package versions listed in what you posted so far.
No promise I will be able to help afterwards but it might help to see the full log, and then again with verbosity turned on. So seperate pastes for:
* `cabal install pandoc --dry-run`
* `cabal install pandoc --dry-run --verbose=2`
* `cabal install pandoc --dry-run --verbose=3`
You might also want to run a `ghc-pkg check` to check to see if your packages are consistent/unbroken.