I am an existing SmartThings user with a ton of custom DTH and SmartApp’s (some self-written / maintained). I was recently prompted in the legacy app to login with my Samsung account. I have verified the username and password I am using as I am able to login from the browser with the same credentials (which then prompt for my 2-factor code); however, from the legacy app on iOS 12 Beta, it always returns that there is a problem with the email or password provided. Not severely a problem for me, but an additional concern is that it has locked in the email address I provided early on for my existing samsung account despite not ever completing the login.
Perhaps the most expedient route would be to contact SmartThings Support. Have you tried that?