This isnt going to help most people, but maybe... My error was occurring on my Windows 10 PC. I installed TT on my Mac (You get both versions if you get the CD), imported the large 1099 on the Mac, and successfully saved on the Mac. So the Mac version doesn't appear to suffer from the bug. Interestingly, post Mac import and save, I moved the file back to my Windows PC and could successfully open and save.
