heya:)
So yes, using old saves will completely wreck the game and you'll get error messages. There's a warning screen when you boot the game up that says not to use old saves and there's definitely a reason for that. That's the most common bug report I get. Basically, if you get an error messages that has "not defined" in it somewhere, that's the old save error. Basically just click new game and then you'll see an "update" spot there. Answer a couple questions which will reconfig your previous choices then it'll drop you at the end of the last update .
And yeah, that google thing is a false warning. it does that with a lot of indie VNs like this