Problem type
Payment required
payment-required
What it means
Two refusals share this slug. With a feature member, a plan gate: the account's plan does not include what was asked for, and plan and required_plans say which plans would. Without it, a write by an account whose payment has failed and which has not yet been moved to the free plan.
What to do
Tell the two apart by feature: with it, a different plan is needed; without it, the outstanding invoice has to be paid. Sign-ins by your users are never refused either way.