The destination realm isn't served by the node that answered (DIAMETER_REALM_NOT_SERVED) -- the request was routed to a realm this agent doesn't know.
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 the Destination-Realm in the request and the realm routing table of the node that answered (often a DRA/IPX agent).
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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