Why do I end up with java.lang.IllegalArgumentException for Casbah / Java MongoDB Driver?
The version of the Java driver you're using is not compatible with MongoDB 2.2. You should be using at least 2.9.3, if not 2.10 or higher. There's no guarantee a driver upgrade will fix your problem, but it should be a first step.
https://support.mongolab.com/entries/22631012-which-drivers-support-mongodb-2-2
How big is your JSON object? See if that big object is supported by the version of mongodb.