Cause
An instant (scheduled_at, starts_at or ends_at) is not RFC 3339 with an explicit zone (Z or ±hh:mm).
What to do
Send the instant with a zone, for example 2026-11-04T15:00:00-03:00.
application/problem+json
{
"type": "https://masnet.com/developers/errors/invalid_instant",
"title": "Invalid instant",
"status": 422,
"detail": "...",
"code": "invalid_instant",
"correlation_id": "..."
}