All relations are lazy in envers, so you need the hibernate session available when you access an object property.
Yes, either that, or pre-access any associations that you'll need (once access they are loaded and subsequent gets don't require the session to be open).
I've figured out the cause of this problem.
BlazeDS tries to serialize objects on the client and server sides. As I can see, envers returns different type of object that is not compatible. The solution in our case was creating the "original" object from the returned one.