I've switched over to cabal for installing xmonad, xmonad-contrib, and xmobar. I thought it would help, and it did for a while, but after updating the the linux kernel just now, the issue reappeared.
However, after recompiling xmonad and restarting my computer, everything is fixed again.
I would suggest that people try a combination of (1) recompiling xmonad and (2) restarting X / restarting computer. Eventually, for me at least, everything gets fixed, and I don't have to worry until next time I update something relevant to xmonad.