I am having the same problem. Surprised they have not fixed it by now. When logging in, the system asks me to change the password, but then I am unable to enter the email, and if I fill in the rest of the fields and submit it errors out.
So I am effectively locked out of my account.
Try going to Marriott.com, hover mouse over SignIn and select the Forgot Password option. Follow that through and you should be able to change the password.