The HSS rejected a Notify from an MME/SGSN because it doesn't recognize that node as the one currently serving this subscriber (DIAMETER_ERROR_UNKNOWN_SERVING_NODE) -- the HSS's own record of who's serving the subscriber doesn't match the node that sent this Notify.
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
Compare the serving node identity in this Notify against what the HSS actually has on record for the subscriber (normally set by a prior Update-Location) -- this points to the HSS's subscriber-location record being out of sync with the network's actual state, worth raising with whoever operates the HSS.
Look at the Origin-Host of the answer: an intermediate agent (DRA/IPX) and the home HSS mean different owners of the problem.
Got a trace with Diameter 5423 DIAMETER_ERROR_UNKNOWN_SERVING_NODE? Drop it in the analyzer: it finds the step that really failed first, which node sent the code and what to ask the other side. Captures, test-system PDFs and vendor exports. 5 free analyses a month, no sign-up.