Broken assassin?

Around 5 days ago my level 32 assassin started saying "Downloadable content corrupted", and now I beat the first play through so I was just starting my second play through, and now when you defeat boom bewm and you're suppose to blow up the gate with claptrap in front of it, it won't blow up, so I'm stuck and it won't continue.

Making a Steam backup always breaks Borderlands 2 for me, and I've tried it on more than one computer and seen others noting this issue too so I am sure it isn't just me... So if I make a Steam backup I have to turn around and do a Steam Restore on the computer I just backed up from. Then it finds some problem and re-downloads whatever backing up messed up in the first place. I don't know if this will help you but if you've made a backup, restore it.