The responding node is congested for this specific APN (APN Congestion) -- a capacity/load issue.
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 with the remote operator whether that APN was under heavy load at the time of the attempt.
Check which node sent the response (MME, SGW or PGW): the cause belongs to that node's decision.
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