Hey there, sorry about the really slow reply, I've been out of the office and this got missed. I completely botched the 1.500 patch for windows in terms of actually forgetting to include the, um, updated executable. So the old version of that executable from unity 3.whatever was not compatible with the new 4.2.1 unity asset files, hence the error you were getting.
I've now updated the update so that anyone else downloading 1.500 won't get that problem anymore. What to do:
1. For anyone reading this and have installed the update after now, then there's nothing for you to worry about.
2. For anyone using a non-windows version of the game, there's nothing for you to do, either.
3. If you are on windows and are having this issue, then you can manually download this file: http://valley2.s3.amazonaws.com/Valley2_1500.zip
Then unzip that into your base install folder for the game, and that will update you properly to 1.500. It will then want you to update to 1.501 after that, but your install is not corrupt or in any way harmed by this. Note: If you want to shortcut this, you can actually even just pull Valley2.exe out of that zip file linked above and replace the version in your install folder with it. That's all that needs to change; everything else is fine.
Sorry about this, and thanks for your patience!