I think version 6.6.1 is the real issue here. I'm not talking about the mod apk, but the vanilla client itself which in turn affects the modded version too.
Excluding my main, I did the usual replace-install on my Y6 and Y3 accounts, and they worked just fine. But since I wanted to test the issue some of us mentioned here, I tried clearing the game data on my Y3 and logging in using the vanilla client (6.6.1). That's when I got the same facebook log in error. I even created a throwaway account and tried linking it to an unused facebook account; same error. Even using ver. 5.2.2 from this thread returned the same result. So either the problem lies with version 6.6.1 or with facebook log in itself. Personally, I'm leaning toward the latter.
Conclusion here: if you only replace-install, you won't get affected, at least that's what happened to me. But, once you log out and try to log in again, you'll get the error.