c# - Invalid ViewState / Invalid character in a Base-64 string

Question

I've read multiple similar posts on this. For example:

• Invalid character in a base-64 string - ViewState?
• Invalid character in a Base-64 string

But none provided the solution for me. I have an idea what is going wrong, and I'll try to walk through my code.

This only happens to a handful of users a day, and all of them use IE. Firefox works as it should.

I have one aspx page which holds a registration form for my webshop. When the user clicks "Proceed," I do a postback which validates data. If everything is valid, I run this code:

public void Proceed(Object sender, EventArgs e)
{
   if (IsValid)
   {
      SaveCustomerInfo();
      ScriptManager.RegisterStartupScript(
          this, this.GetType(),
          "RedirectPaymentGateway",
          "RedirectPaymentGateway('" + paymentData + "');", true);
   }
}

This script redirects the user to the payment gateway like this:

function RedirectPaymentGateway(paymentData) {
    $(document).ready(function() {
        try {
            var div = document.createElement('div');
            div.innerHTML = paymentData;                
            var formDataObj = jQuery.parseJSON(div.firstChild.nodeValue);

            var form = "<form>FORM CONSTRUCTION</form>";

            $('#pnlPaymentGateway').html(form);
            $('#pnlPaymentGateway').submit();
        }
        catch (e) {
            alert(e);
        }            
    });
}

(paymentData is a JSON serialization of a .net object.)

I'm guessing this code breaks something related to the viewstate, but I can't figure out how or why. Any suggestions?