I'm unable to load chapter 'Brainless Invassion', after finishing 'Terror in the city', when I was playing the first level I decided to quit, and when I came back the chapter was disabled, I had to play the level 'The incredible Gertrude' again to reach next chapter.

I'm having the same problem

This bug seems to have been fixed in the latest version that's still missing on GOG

Still no update. It seems that we are still seven updates (1.3.1 released on 25.10. - 1.3.7 released on 10.11.) behind. The bug was fixed in 1.3.4.

It looks like we have an update now. Do you know the best way to install updates without nuking saves? I have completed levels and challenges that I don't want to have go away.

I'm on Linux. The save games and progress information is stored in $HOME/.config/unity3d/NoClip/Zombie Night Terror, i.e. outside the game folder. I would assume that it's the same on other operation systems as well. If you want to be on the safe side you could search for the NoClip folder. On Windows >= 8 it should be in your user (usually c:\users\username) folder. You might even make a copy of that folder if you want to be extra safe.

It this case it would be the best to uninstall the old version and afterwards install the new version.

That was precisely my plan! Thanks! (I have Windows 7, but I can still look for the NoClip folder with a command prompt and dir/s. Knowing what to look for was my problem.)

EDITED TO ADD: And it looks like we got version 1.3.8.