I confirm the error is due to incompatibility. Server version 7 requires client libraries (Android or iOS) version 4.
A full tutorial for the new MPN client APIs is available on our blog here: Mobile Push Notifications with Lightstreamer Server 7.0 and Client SDKs 4.0. Feel free to ask if you need any help porting your code to the new APIs.
BTW, are you the same Lazar that contacted us on our support e-mail address? If this is the case, I will continue the discussion there.