The request's message format itself was invalid (Invalid Message Format) -- a protocol encoding problem, not a policy rejection.
Where you see it
In the Cause IE of a GTPv2-C response (Create Session, Modify Bearer, Delete Session…) on S11, S5 or S8.
What to check
Report to whoever implements the sending node -- this points to a malformed message, not a business-logic issue.
Check which node sent the response (MME, SGW or PGW): the cause belongs to that node's decision.
Got a trace with GTPv2 cause 65 Invalid Message Format? 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.