So, it looks like there's two issues:

1. Voxel parsing isn't threadsafe
2. The saving process creates a lock file, but it's not removed in the case of an error.

As a workaround, if this happens again, you should be able to delete the "save.lock" file in the save folder to resume auto-saving.

Ok, so here's what's going on:
I created a separate ticket, 0000289, to handle the InvalidOperationException thrown during save. This issue is much more complicated to resolve, and probably won't make it in the next hotfix.

However, the second issue, where auto-saves break, that's a much simpler issue to resolve. So I did.
The exception will still occur, however while the current save will fail, auto-saves will still continue working, so the next save should be successful.
At least this way if and when it occurs again, potentially hours of work won't be lost.