Brightly lit, with torches around.
Dropping a bucket of water onto the field and popping off every seed and then replanting seemed to "fix" it, strangely enough. Magic. Couldn't reproduce it afterwards, either.
They do so in Singleplayer, too. Just trying to place on crashes the game for me.
My game keeps crashing, too. As soon as I look at an elaborate redstone contraption(hidden by other bocks) it crashes on me. If I am fast enough, I can turn around, so I do not look at those particular chunks, but as soon as I turn back...crash.