Error after move to Windows 2012 R2 server

Hi,
I have to identical websites with a custom made membership control based on the default ASP.NET membership components. I moved it to a new Windows 2012 R2 server. It is an exact copy of the old website which was on Windows 2008 R2.  Based on my investigation everything on IIS is configured identical on the new server. Everything on the website works perfectly, except on little thing. When the users try to activate their account this error is prompted on the new web server. The old server, with exact the same code works fine.
Stack Trace: [NullReferenceException: Object reference not set to an instance of an object.] MyServices.Membership.WXMembership.EncodePassword(String sPassword) +171 MyServices.Membership.WXMembership.Activation(String sUsername, String sNewPassword, String sValidationCode) +918 welcome.Validation() in C:\admin1\welcome.aspx.vb:64 welcome.btnValidate_Click(Object sender, EventArgs e ...