I have gotten that error when I was saving a business object with multiple child business objects and they saved in an order that broke SQL Servers constraints. Has there been any changes to the DB that would cause one or more constraints to fail?

what happened in my case is that on of the child bo's failed to save and then the next bo fired its save and the transaction had closed due to the first failure.
To catch it I turned the autosave on parent save to false and called each one of the child saves individually and checked the save result. Once I found the offending BO I just changed the order it saved in.