При попытке нажать кнопку Войти (уже после самой регистрации) получаю:

Invalid URI: The URI scheme is not valid.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.UriFormatException: Invalid URI: The URI scheme is not valid.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[UriFormatException: Invalid URI: The URI scheme is not valid.]
System.Uri.Parse() +2271
System.Uri..ctor(String uriString, Boolean dontEscape) +184
System.Uri..ctor(String uriString) +7
System.Web.HttpRequest.get_UrlReferrer() +164
ActualForum.login.Page_Load(Object sender, EventArgs e) +196
System.Web.UI.Control.OnLoad(EventArgs e) +67
System.Web.UI.Control.LoadRecursive() +29
System.Web.UI.Page.ProcessRequestMain() +724

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Version Information: Microsoft .NET Framework Version:1.0.3705.288; ASP.NET Version:1.0.3705.288