EMM cause (LTE NAS)3GPP TS 24.301 §9.9.3.9 and Annex A
What it means
UE security capabilities mismatch: the security capabilities echoed back by the network don't match what the phone sent.
Where you see it
In LTE NAS Attach reject, Tracking area update reject, Service reject or Detach request.
What to check
Check the eNB/MME handling of UE security capabilities.
Look at the S6a (MME↔HSS) and S11 messages just before the reject: they usually show the real trigger.
Got a trace with EMM cause #23 UE security capabilities mismatch? Drop it in the analyzer: it finds the step that really failed first, which node sent the code and what to ask the other side. Captures, test-system PDFs and vendor exports. 5 free analyses a month, no sign-up.