Switch the firmware to braiins OS and do the suggested above again.
The suggested above should be work because bitmain has a guide that you need to test the hashboard one by one so it means it should run even one hashboard. You might be have a faulty hashboard that is why you are getting error when running one hashboard. Try to use Braiins OS or check your Ethernet connection.
S9's should run fine on any number of boards, suggest testing each one in turn to find the dead one.