http://localhost:8080/openid/login is trying to log you in via single-sign-on, so I don't think that would work locally.
Usually there is a page where you can login directly in de app, like http://localhost:8080/login.html or http://localhost:8080/index3.html. you can look in the theme folder of the project directory which html files you have in your application.
I was able to fix it for my app: in Mendix Studio Pro, go to Security and make sure the Security is switched to Off or Prototype.
Note the trick to run a blank app did not work for me: it runs that app fine but this one was not happy till i switched back to no security.
I’m not sure if you how/if you can run SSO with a locally served app in test mode.