Martin,
That error code means the mac layer is busy with the previous transmission. If your device has TRACE logging turned on, you should see the following from the debug port: “Link is busy with previous send”
One slave wouldn’t know if it was transmitting over another slave or not, so that’s not the issue. This error usually happens when attempting to transmit too soon after the previous transmission.
1s is not a lot of time to TX a packet and RX an ACK, depending on the datarate and frequency band. Does the issue go away if you increase the interval between TX for the slaves?
Cheers,
Mike