.NET Forms Authentication Doesn't Work

I have an issue with my .NET application and it can only be reproduced sometimes and only on client's location, which makes it extremely difficult to resolve. In the application the forms authentication is used. There is a login page to which every anonymous user is redirected. After user enters username and password, it redirects to DefaultURL or some predefined page. The problem is that sometimes when user enters correct username and password, it doesn't login, instead it just refreshes the login page without any error messages. After several tries of inputing username and password, user is able to login to the system.

See my login page Submit_Click code below:

Dim userName As String = tbUsername.Text
Dim password As String = tbPassword.Text
Dim returnUrl As String = Request("returnUrl")

[code]....