STP Help

Hello All!

I am primarily a Server/OS admin recently thrown into the fire with a networking project. We recently purchased a few Cisco Catalyst switches and I am configuring trunking between the new switches and some older Avaya ERS 3500 switches. I have the new switches (let's call them "Switch A" and "Switch B") configured in a stack and am trying to redundantly connect them to an Avaya switch "Switch C".

So I configured 2 port channels, one on Switch A and one on Switch B. Both port channels are in trunk mode and meant to be connected to Switch C. On Switch C I configured two multi-link trunks connected respectively to the port channels.

I connected the first port-channel with two cables, everything looks great. I then connect the 3rd cable for the second port-channel and it gets blocked by STP. Good, understandable as the switches are in a stack. And then here comes the problem. I plug in the 4th cable and the whole network goes down. I can't even get to Switch C when I am directly connected to it. I unplug the 4th cable and everything goes back to normal.

I obviously suspect a loop, but I am confused as to why. It seems I am missing something obvious. Any insight you can throw my way would be appreciated. Sorry for the noob question!