Another strangeness here.
I have a file that opens and saves fine with db '2015-06-20 1.1 legacy hotfix' (which I worked on with) but will not save with any of the newer builds. It just stalls when it creates the temp max file, leaving it at 32kb. The file didn't save even after 30 minutes of waiting. What's interesting is that the file takes quite long to open with the newer build, whereas it opens normally with the older build.
There's nothing special about that file, the only thing that stands out is thousands of instanced objects using CoronaMultiMap and a CoronaScatter object... I really can't think of anything else, and trying to reproduce didn't work. Sorry for the vague description but the fact that it behaves this way with the new builds only (starting from 17 Jul) may help.
EDIT
Merging into a new file seems to help, saving works and is much faster now.