The charging server doesn't know this subscriber (DIAMETER_USER_UNKNOWN, RFC 4006).
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 that the subscriber exists in the OCS/charging system.
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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