The actual data are:
Just by chance I discovered that a SIM pin change solves the problem, which however is not quite convenient, as it is required upon every system start.
Basically the phone starts, no SIM pin request comes up and hence does not register the SIM at the network (IMEI shows correctly upon *#06') AND there is no way to add an APN (just does not save any new entry).
As soon as a SIM pin change has been confirmed by the system it registers at the network and works fine until restart.
Do you know any cure for such behavior?