title: Azure AD B2C 'Invalid username or password' when using built-in policies
body: <p>I have setup an Azure AD B2C tenant and created an application registration. I am using local accounts only. I created two users a few months ago. Today, I tried to sign-in using these two users using the built in 'SignUpSignIn' flow. However, I keep getting the error "Invalid username or password", even though the user exists in my tenant and has the right username. I even tried to add the user again to the tenant using the signup flow, but, correctly received the 'this username already exists' error.</p>
  
  <p>I looked up this error on SO and noticed that folks have run into this issue when using Custom Policies. However, I do not have any custom policies. I'm just using the built-in policies.</p>
  
  <p>Just to confirm that it is only a problem with existing users, I created a new user and tried to reproduce the error. However, I am not having the same issues with this new user. I am able to login without any problems with this new user. The problem only seems to be happening with the users I have previously created. </p>
  
  <p>Can anybody please guide me in the right direction? Thanks!</p>
  
  <p><a href="https://i.stack.imgur.com/ibCSc.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ibCSc.png" alt="enter image description here"></a></p>