Build a full-stack app using MongoDB Realm GraphQL (without worrying about servers at all)

A little late here but if the json(status 400) error you were referring to applied to the sign up form. I resolved this by making a small change to the function.

const emailPasswordSignup = async (email, password) => {
        try {
            await app.emailPasswordAuth.registerUser({email, password});
            return emailPasswordLogin(email, password);
        } catch (error) {
            throw error;
        }
    }