CONNECTION_UNAVAILABLE

The data source had no connection to give, rather than failing to open one - a pool that ran out of time waiting for a free one, most often.

Nothing reached the server. The database and the credentials are not what is in question here; there was simply nothing free at that moment, which is a fact about the application's own demand for connections. details carries what the data source said, which for a pool includes its own count of how many were in play.