"Timeout waiting for an available tube" from Dax

Timeout waiting for an available tube indicates your client cannot establish a connection with DAX. So ways to troubleshoot the issue would be to investigate your DAX Metrics such as ClientConnections, CPU Usage etc.. to see if you can spot any trend there. Timeouts like this can commonly happen from resource contention such as creating too many connections concurrently.

Other things you can check is your network path from your client to DAX, are you seeing any other timeouts or can you ping DAX at the time of a timeout to see if it is reachable for example.