Well, the bottom nut fought to the last thread as well
It's similar to what you describe diesel911. I don't think it was cross threaded and the female threads look OK. At least it didn't pull the threads off
I think it's that either corrosion set up between the steel and aluminum or the threads on the cooler weren't machined correctly. I'm thinking the latter...
I believe I can get it back together OK. There's a lot of thread, it's just a little buggered. I've straightened out worse with a thread file. It has some kind of bubble flare so hopefully it won't require a lot of torque to seal.