AWS IoT not working on two devices

[Martinnn]

I have set up a handful of WiPy boards to publish data on AWS IoT. I followed the example for the pycom AWS library and standalone, it works. I send a JSON string every 2 s via wifi to an LTE router. In AWS IoT the messages are routed to a DynamoDB. Each board has separate keys which are selected based on the unique ID. On its own, each board works.

However, when I connect two boards to the same router, only one transmits data to AWS IoT. The other gives
Socket receive error: -80
Socket send error [Errno 104] ECONNRESET
repeatedly.

Now best of all is that this might move from one board to the other - after a while the one with error starts transmitting, while the error occurs on the board previously working (without restart or reconnect). In the AWS IoT MQTT client I see a "last will" message and the reception switches to the other board.
Did anyone manage to use the pycom AWS library for two or more devices simultaneously?

[Martinnn]

... forgot duplicate CLIENT_ID in config.py...
Problem solved!