The request went round in circles between Diameter agents (DIAMETER_LOOP_DETECTED) -- a routing configuration loop.
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 Route-Record AVPs in the request to see which agents it passed through, and fix their routing tables.
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 3005 DIAMETER_LOOP_DETECTED? 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.