The peer doesn't support the Diameter application this request used (DIAMETER_APPLICATION_UNSUPPORTED) -- e.g. S6a isn't enabled on that peer connection.
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 right application (e.g. S6a 16777251) was agreed in the CER/CEA between the two peers.
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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