Have you tried it with other files? It might be some particular elements or conditions in the file causing the problem.
If the problem is with all files then you should try another machine if possible.
If other files do work (you can try just a single wall) the next thing would be to try saving the file story by story. The easiest way to do this would be to hotlink into a new file one story at a time (be sure to check "single story mode" in the dialog) and save to 12 from there.
If you can isolate a problem story then do a binary search for the problem parts. To do this would require breaking the link and deleting elements by halves (if you delete half the stuff and the file saves then the problem is in the other half, etc.). This is probably best done by layers.
