Well, Valve's fix to stop the engine errors did nothing at all. Payload and Votation are still suffering from the same crashes. We're going to try to keep these crashes to a minimum, but unfortunately, Valve holds the key to this mess... and they're gone for the weekend.
Keep an eye on these forums, we'll keep you guys updated. The MvM servers are stable though, I guess that's something. Sorry again for the inconvenience... I wish Valve did some actual QA instead of having their customers do it for them.