Hey André,
I've seen issues like that on previous releases, but all of them have been fixed. The difference you see between the StoreOnce 4500 and StoreOnceSoftware configuration might be related to the additional compute requirements of the StoreOnceSoftware service in addition to the regular Media agent activity. Is the system heavily loaded during the object copy? If yes, try to reduce concurreny on the destination device. If serializer still crashs, collect debugs and a full crash dump of RMA. This should be easy to capture.
Regards,
Sebastian Koehler