Loginasync does not execute for certain users

I have an .NET application written in C#. I’m using the LoginAsync function using Username_Password_Authentication. For whatever reasons, for one of the user accounts, it freezes and never returns LoginResult. For other users it works perfectly fine. I tried recreating the account with a new user_id(auth0|#)to no avail. In addition… I tried adding new users. for anyuser that has user@<custom_domain> doesn’t work. However, if it is user@<gmail.com,live.com,etc> it works fine. Any existing user@<Custom_Domain> works as well. Just any new user with user@<Custom_Domain> has the issue of LoginSync freezing and not returning LoginResult.