There are a couple of crash bugs.
This is the best description of one, and a way to handle it, that I've read, and yours sounds at least nominally similar -
http://www.bay12forums.com/smf/index.php?topic=98077.msg2856486#msg2856486In the development log shortly after that tale was posted, Toady commented that he'd fixed a crash bug related to forgotten beasts, one that he was sure had scuttled countless fortresses.
I've had a map that I had really enjoyed, until it started to crash on a specific day in play. I tried 8 times to get past that day before I gave up.
The only comfort I can offer you - crash bugs like this really do seem to be rare for most players. I've only had one map affected by it to date. When bugs like this are mentioned, I've seen several players seem skeptical - that's perfect. That means they've never had such a crash-bug, so it's even more rare! Condolences on having gotten it twice in two games, may you never see it again.
edited to add:
That said, I was curious and downloaded your save - when is it crashing? Right when you load it?
For me it will open and run. I revealed and found one forgotten beast, watched it meander at a fairly good speed, it didn't seem to be doing anything but walking and the game did not crash for me - though I only let it run about a month.
Considering this, I recommend you try a new installation of DF (with or without those mods, once the save is created it carries its raws, and graphics (if changed) are the only things that wont move as a unit). Just drop a copy of the save in that and see if you still have troubles?