At one location I have 3 WS-C3524-XL with IOS version: 12.0(5.4)WC(1) switches and 1 WS-C3550-24 with IOS version 12.1(13)EA1a) switch that work together nicely. They are all connected with GigaStacks. When I add number 5, also a WS-C3550-24 but with a different IOS version: 12.1(19)EA1c all connections are lost between the switches.
What is going wrong?