Facing Bugs in Production sntaxError: Unexpected end of JSON input

I hosted a Chat-App builded using Next JS that, Just having Authentication Right now, throwing an error syntax error: Unexpected end of JSON input

• In My localhost ( development server same code works fine )
• I even tried it through local host build server

When I hosted the app on Vercel , After setup the environment Variables properly and could see the error i mentioned above, Now I have no Ideas why its throwing an error if its fine in local host

Here is My Code

Services Page

export const register_user = async (formData) => {
    try {
        const res = await fetch(`${process.env.NEXT_PUBLIC_API_BASE_URL}/api/auth/register`, {
            method: 'POST',
            body: formData,
        })
        const data = await  res.json();
        console.log(res)
        return data
    } catch (error) {
        console.error('error in register_user: (Services) => ', error)
    }
}



export const login_user = async (formData) => {
    try {
        const res = await fetch(`${process.env.NEXT_PUBLIC_API_BASE_URL}/api/auth/login`, {
            method: 'POST',
            headers: {
                'Content-Type': 'application/json'
            },
            body: JSON.stringify(formData)
        })
        const data = await res.json()
        console.log(res)
        return data
    } catch (error) {
        console.error('error in login_user: (Services) => ', error)
    }
}

export const reset_user_password = async (formData) => {
    try {
        const res = await fetch(`${process.env.NEXT_PUBLIC_API_BASE_URL}/api/auth/resetPassword`, {
            method: 'POST',
            headers: {
                'Content-Type': 'application/json'
            },
            body: JSON.stringify(formData)
        })
        const data = await  res.json();
        return data;
    } catch (error) {
        console.error('error in login_user: (Services) => ', error)
    }
}

Backend Code

import ConnectDB from '@/DB/connectDB';
import User from '@/models/User';
import Joi from 'joi';
import { compare } from 'bcryptjs';
import jwt from 'jsonwebtoken';


const schema = Joi.object({
    email: Joi.string().email().required(),
    password: Joi.string().required(),
});




export default async (req, res) => {
    await ConnectDB();

    const { email, password } = req.body;

    const { error } = schema.validate({ email, password });

    if (error) return res.status(401).json({ success: false, message: error.details[0].message.replace(/['"]+/g, '') });

    try {
        const checkUser = await User.findOne({ email });
        if (!checkUser) return res.status(401).json({ success: false, message: "Account not Found" });

        const isMatch = await compare(password, checkUser.password);
        if (!isMatch) return res.status(401).json({ success: false, message: "Incorrect Password" });

        const token = jwt.sign({ id: checkUser._id, email: checkUser.email }, process.env.JWT_SECREAT, { expiresIn: '1d' });
        const user = {name : checkUser.name , photo : checkUser.photo, _id : checkUser._id , email : checkUser.email}
        const finalData = {token , user }
        return res.status(200).json({ success: true, message: "Login Successfull",  finalData})

    } catch (error) {
        console.log('Error in register (server) => ', error);
        return res.status(500).json({ success: false, message: "Something Went Wrong Please Retry Later !" })
    }
}