Firstly: It sounds like a bug on Apple's part. Core Data is calling _Unwind_Resume, which is (probably) some sort of exception unwind. Exception-unwinding exists on the phone, but (I think) uses the ARM ABI, which uses function names beginning with __cxa_. Are you running on the simulator? Which version of the SDK?
There might be an extra release floating around somewhere which is "balanced" when you remove the call to [account release];.
"Instruments doesn't show any leaks" doesn't mean there aren't any; last I checked it got confused by cycles (i.e. it wouldn't show a leak if you forgot to un-set IBOutlets in dealloc). I tested with NSMutableData * d = [NSMutableData dataWithLength:1<<20]; memcpy(d.mutableBytes, &d, 4);, but an easier test is just [[UIView alloc] initWithFrame:CGRectZero].
If you think it's a retain/release issue, I once debugged these by overriding retain/release/autorelease to call NSLog. I then added breakpoints on all of them, set them to run the command "bt", and clicked the autocontinue. Then run the thing that breaks (in my case I think it was just an extra retain), print out the log output,stick it on a whiteboard, and spend half an hour matching retains and releases.