Devices can't connect to IoT Core

Hello, My devices can't establish mqtt connection to IoT Core in Oregon. ioT Core reports connection lost.

{ "clientId": "", "timestamp": 1639587896509, "eventType": "disconnected", "clientInitiatedDisconnect": false, "sessionIdentifier": "", "principalIdentifier": "", "disconnectReason": "CONNECTION_LOST", "versionNumber": 64 }

Any idea what is going on?