This seems to be happening every time I start a new family now (I do have World Adventures), I start a new family, it saves fine, I exit, and when I return I can't load the neighbourhood because "a serious error has occured... try restarting", but when I try restarting, I just get the same message. I tried restoring the backup, but that doesn't work either. Any suggestions what may be causing this?

You have removed folders/files with .bad extensions?
When you say "every time I start a new family now", is this on the same game or a new game?
And it occurs only for a single town or both (SunsetValley & Riverview)? Or on a new .world?