The peer has no record of this Session-Id (DIAMETER_UNKNOWN_SESSION_ID) -- e.g. a CCR-Update for a session it already closed.
Where you see it
In the Result-Code or Experimental-Result-Code AVP of a Diameter answer (S6a, S6d, Gx, Gy, Cx…).
What to check
Check whether the session was terminated or lost on the server side earlier.
Look at the Origin-Host of the answer: an intermediate agent (DRA/IPX) and the home HSS mean different owners of the problem.
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