What operating system? Server=Windows Server 2003 / SP3 / IIS 6; Client=Windows 7 / SP1
What database platform? MS SQL 2008
What version of mojoPortal? Upgrading from 2.3.9.4 to 2.3.9.7
What version of .NET? 4.0
Steps to produce the problem, expected results, and actual results.

On previous version (2.3.9.4) we created a custom page not found, per the instructions (https://www.mojoportal.com/customizing-the-404-page-not-found-page) putting our own text and links inside the main <div>. After upgrading to 2.3.9.7 the custom page is throwing errors - "XML Parsing Error: no element found". But we can hit the included/default pagenotfound.aspx. So we copied our custom text over and pagenotfound.aspx throws errors in IIS about unhandled exception, NullReferenceException, and Object reference not set to an instance of an object.

Upon narrowing it down, it seems there is a change such that it is now required to have this line: <asp:Literal ID="litErrorMessage" runat="server" EnableViewState="false" />. Whenever we take that out, we get the errors. However, previously that was not the case. Seems that forces us to keep the default error message/text rather than be able to customize the page.

Thanks in advance!

Joe, thanks that does help. Didn't think of that... =) It might be a good idea to update the custom page not found instructions (https://www.mojoportal.com/customizing-the-404-page-not-found-page) with the requirement for that asp literal line to stay in.

One more thing on our custom page, before upgrading we created a link to the Site Map using: <asp:HyperLink ID="lnkSiteMap" runat="server" CssClass="txterror pnflink" />. That's not working anymore now; just doesn't render at all.