Well, that's troubling. I could ascribe once to mere coincidence (I know there's some kind of memory corruption bug left in the server that I still haven't managed to track down that strikes every once in a blue moon), but when it has happened twice, there's clearly some systemic problem.
I have no clue what causes this, but fortunately there is some redundancy in the internal map representation, so I have now added a validation check that runs every time a map is saved, so the server will at least crash instead of unknowingly saving a corrupted map in the future, should it ever happen again. Until such time as I manage to find the problem, this should at least be better.
Unfortunately, there's no way to restore the map. My condolences. As with the last time this happened, I'll make sure to remove any left-over inaccessible claims and stuff. Tell me if there's anything I miss.