okay, so i managed to track down what is causing the error.
i was able to piece back my file from a slightly out of date backup file and the audio data... just a matter of finding what was missing and putting it back into place.
then later on when i was editing something, i used the time stretch plugin and i got the same error as before.
so i loaded it up again, did some more editing, saved a few times successfully, then i needed to stretch a note in one of the tracks. it stretched fine, but then when i tried to save it i got the same crash again!
so i guess that means i've narrowed it down to that plugin, which i will simply avoid using unless anyone has any suggestions.