I made a few changes in the class definition and it worked for me. It is important to keep in mind that the Bool and Int properties must be initialized, if they are declared optional, serialization of the values does not properly work. Here are my property definitions:
var objectId: String?
var ownerId: String?
var created: NSDate?
var members: [BackendlessUser]?
var suhCount: Int = 0;
var sendPush: Bool = false;
var lastSent: NSDate?
var lastSender: BackendlessUser?
var group: Bool = false;
Also, check the type of the “members” and the “lastSender” properties in Backendless console. If they were created dynamically, I suspect they will have type STRING in console - make sure they are declared as relations.