1. Is that a mass problem?
2. SAU limit on attach in SGSN not exceeded?
3. Can any another subscriber from the same HLR attach to that SGSN?
4. Can You capture SS7-trace of AUTH,LU,ISD sessions btween SGSN and HLR? Maybe it’s a MAP version negotiation conflict between SGSN and HLR. Anyway it’s interesting which transaction failed.