As soon as you pick a venue in Venues Location, Sprwt tries to run a quick availability check in the background. In the current version this quick check has a bug: it uses the venue's own ID as if it were the proposal's ID to look up a record, so on a new proposal (or whenever that ID doesn't match an existing proposal) it responds with Invalid Proposal ID!; if it does find a record but that record has no saved event time, it responds with Insert Event Time! instead. Neither message means your entry is wrong, and when the venue actually is available this check shows no confirmation at all — it only ever displays an error. The check that actually blocks a double booking runs when you click Save, so fill in the times, save, and rely on that result instead.