Authentication failures (T3260 expires)

Any core guru here? I have detected an abnormal behavior of some users in my network (3G RAN and core from Ericsson). Certain users seem to get stuck in the registration procedure. I have seen repeatitive location updates failures since the UE does not answer to the authentication request from the network leading to a radio connection release after timer T3260 (12 seconds) expires. At the radio level everything works ok, the failure happen at the NAS protocol level. For instance there was an user with 400 location update failures in 24 hours all of them with the same reason, the UE does not send the authentication response. I have seen this problem in phone models from different vendors (Nokia, Sony Ericsson, Samsung…)