The responding node only supports GTPv1, not GTPv2, for this procedure (Fallback to GTPv1).
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
Confirm which GTP version the responding node's interface actually supports.
Check which node sent the response (MME, SGW or PGW): the cause belongs to that node's decision.
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