Why is one half of a subpanel not working?
Is anything connected to the 'red' side? I am not 100% clear on what is connected to the subpanel but the debugging sequence should be the following:


turn off all the breakers on the 'red' side of the subpanel. Check if the main breaker still trips. If it does not, the issue is with whatever hooked up to that side (shorted wire, etc) 
If it still does, swap with the 'black' breaker to see if the breaker itself is the problem. 
If the problem continues, check the wiring.