subscription_version_conflict
HTTP 409, type conflict_error.
The subscription was changed by another request while this one ran.
Why it happened
Two writes reached the same subscription at once, and this one lost the race, so it was not applied.
How to fix it
Read the subscription again, then retry the change if it still applies.
Retrying: send the retry with a new Idempotency-Key. The conflict is detected while the request is processed, after its key is claimed, so the same key will not run it again.