I have the same problem. I have tried lot of things allready - cable swapping, upgrade, downgrade. Problem is a half-duplex stackwise operation. It can be seen by the "show switch stack-ring speed". One pair of the ports are down and the others are on half-duplex.
I have just tried to connect third 3750 (older one). The result is - the third switch has always both ports up and the others have always down the second port. I tried to reconfigure the ring and the result is the same. Bad switches have always one port up and the second one down.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...