The peer received an AVP it doesn't support and that was marked mandatory (DIAMETER_AVP_UNSUPPORTED) -- an encoding/version mismatch.
In the Result-Code or Experimental-Result-Code AVP of a Diameter answer (S6a, S6d, Gx, Gy, Cx…).
Careful: 5001 also means DIAMETER_ERROR_USER_UNKNOWN. In Diameter the base protocol (Result-Code) and 3GPP applications (Experimental-Result-Code) reuse the same numbers, so read the name, not just the number.
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