Change context path to index page, but show status 404

Question

I have a similar issue with this post that I get status 404 instead of index page by adding context path. I doubt that I did something wrong like put the @EnableWebSecurity in the WebSecurityConfig file or in the Controller.

Here is the spring-boot config

@Override
protected void configure(HttpSecurity http) throws Exception {
    http.authorizeRequests().antMatchers(
            "/registration**",
            "/js/**",
            "/css/**",
            "/img/**").permitAll()
            .anyRequest().authenticated()
            .and()
            .formLogin()
            .loginPage("/login")
            .failureUrl("/bad-404")
            .defaultSuccessUrl("/")
            .usernameParameter("email") //needed, if custom login page
            .passwordParameter("password") //needed, if custom login page                
            .permitAll()
            .and()
            .logout()
            .invalidateHttpSession(true)
            .clearAuthentication(true)
            .logoutRequestMatcher(new AntPathRequestMatcher("/logout"))
            .logoutSuccessUrl("/login?logout")
            .permitAll();

}

Here is the main class to run the app

public static void main(String[] args) {
    System.setProperty("server.servlet.context-path", "/index");
    SpringApplication.run(SmartcardApplication.class, args);
}

Here is the Controller class

@Controller
public class MainController {

    @GetMapping("/login")
    public String login() {
        return "login";
    }

    @GetMapping("/")
    public String home(){
        return "index";
    }
}