I downloaded the latest version (from mod.io, the latest release on github still not updated) and got this error messages. But everything's fine. I'm not sure what is actually happens.
I played Brits and Cunobelin's issue is no more, as Brits Fortress is working too. Thank you, @Grapjas!
PS: actually, I have tried this too:
I used the version 3.1.3 (with Cunobelin's issue) and replace hero_cunobelin.xml with the fixed version. And the same error as above also appears. So I think the fix for issue #19 itself makes this error. And honestly, I don't know what the error means, because I don't encounter any bug or any strange thing while playing the game. (At least for now :V)