A required field was missing from the request entirely (Mandatory IE missing).
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
Check the exact request contents against 3GPP TS 29.274 for this message type.
Check which node sent the response (MME, SGW or PGW): the cause belongs to that node's decision.
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