One of the traffic files you have is corrupt, so you need to find out which one.
The easiest way to do that is to remove a group of them, the most recent 20 for instance. Just copy them to a temporary folder and loading the sim again using the same time, day, and airport as where it was stalling at 77%. If it loads, then you know one of those files that you removed is the one. From there, shut down the sim and add a few of the suspect traffic files back in and try again, but it's very important to keep track of which ones you add. Eventually, you'll find the corrupt one.
I went through this a few times last year and it really only takes about 30 mins to track down.