A given user attempts to sign-in to Office 365 with their Azure AD Premium account and password. When they do, they get "We don't recognize this user ID or password" error even though, they are using the correct user name and password. Attempts for them to reset their password is not possible since they cannot sign-in.
One observation, is that their account has capital letters in it like John.Smith@contoso.onmicrosoft.com in Azure AD. when the user did try to sign-in, they signed in with firstname.lastname@example.org and failed. From research, i see nothing about case sensitivity with accounts. So, am assuming there is something else going on here.