Why is ScriptResource throwing Unknown Runtime Error on ReportViewer 12 drilldown?

Question

Currently migrating an ASP.NET website which has embedded ReportViewer controls in the pages. The report shows up fine in the viewer, but when we click any drilldown link, IE indicates an "Unknown runtime error" in ScriptResource.axd. We are using ReportViewer 12, but had the same issue with ReportViewer 10; we thought upgrading to 12 would solve this, but it did not. I found a similar issue with updatePanel, but it was not helpful. I can post any additional information on request.

Error Message

Message: Unknown runtime error
Line: 1806
Char: 9
Code: 0
URI: http://{server-hostname}/{Path}/ScriptResource.axd?d=HURY6fWJG979L4dvjF7qlfDEC
Jny9VK-KMhlGWqyXlmaGtN_PSJwOijkOw__2eElLRRrfm_T1O2eGEITZnlywp
7rZLCcaeFP2C6YzKTzdcARY90AxK-TYzamRaw54Sjz1HmSjcvT
s_AbkVas-O2rxil2mMnRtfFW-zZTjHFDn0tcXJEiDnEzUdLZMJ
_oU2xf0&t=72e85ccd

web.config assembly references

<add assembly="Microsoft.ReportViewer.Common, Version=11.0.0.0, Culture=neutral, PublicKeyToken=89845DCD8080CC91"/>
<add assembly="Microsoft.ReportViewer.WebForms, Version=11.0.0.0, Culture=neutral, PublicKeyToken=89845DCD8080CC91"/>

Page Source

<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true"
CodeFile="Dashboard.aspx.cs" Inherits="Dashboard" Title="Dashboard" %>

<%@ Register Assembly="Microsoft.ReportViewer.WebForms, Version=11.0.0.0, Culture=neutral, PublicKeyToken=89845DCD8080CC91"
Namespace="Microsoft.Reporting.WebForms" TagPrefix="rsweb" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" runat="Server">
    <div>
        <p>
            <rsweb:ReportViewer ID="rvDashboardMain" runat="server" Font-Names="Verdana" Font-Size="8pt"
                ProcessingMode="Remote" EnableViewState="true" Width="95%" CssClass="rvFont"
                ShowBackButton="true" ShowParameterPrompts="True" PromptAreaCollapsed="true"  Height="520px">
                <ServerReport ReportPath="/{my-report-path}/{my-report}" />
            </rsweb:ReportViewer>
        </p>
    </div>
</asp:Content>

Code back page

public partial class Dashboard : System.Web.UI.Page {

protected void Page_Init(object sender, EventArgs e) {

    if (!Page.IsPostBack) {

        ServerReport serverReport = rvDashboardMain.ServerReport;        
        serverReport.ReportServerUrl = new Uri(ConfigurationManager.AppSettings["ReportServer"]);
    }    
}

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        string UserID = HttpContext.Current.User.Identity.Name;
        UserID = UserID.Substring(UserID.IndexOf(@"\") + 1, UserID.Length - UserID.IndexOf(@"\") - 1);
    }
}

protected void rvReports_Drillthrough(object sender, Microsoft.Reporting.WebForms.DrillthroughEventArgs e)
{
    ServerReport rpt = (ServerReport)e.Report;

    rvDashboardMain.ServerReport.Refresh();
}
}

Anchor for the drillthrough link

<a 
tabindex="26"
onclick="var rp=$get('ctl00_ContentPlaceHolder1_rvDashboardMain_ctl09_ReportControl');if(rp&amp;&amp;rp.control)rp.control.InvokeReportAction('Drillthrough','188iT0R0x0:0');return false;" onkeypress="if(event.keyCode == 13 || event.which == 13){var rp=$get('ctl00_ContentPlaceHolder1_rvDashboardMain_ctl09_ReportControl');if(rp&amp;&amp;rp.control)rp.control.InvokeReportAction('Drillthrough','188iT0R0x0:0');}return false;" 
href="" 
style="cursor:pointer;" 
class="A1d3b265b4e26456c97f9dd1067b6eec2186a" 
target="_top">2,551</a>