I created an app in cuba 7.2.6 and also added the front module. I can’t log into the app-front module with the normal admin/admin credentials. I receive a “Login Failed” error. In the normal app module i can login with these credentials.
According to our new security policy it is necessary to provide acess for screens/entities/add-ons functionality. In your case you have to create a new role with REST Security scope and allow Use REST API permission in the Specific tab. After that assign this role to admin user and everything should works fine.