You are making some redirect and you are changing the HTTP request. If you take a look at the screenshot you have attached you will see that the request is made to htm file located in your file system "C" drive.
Somehow you are changing the ICmsUrlContext and you are passing noting to the UrlHelper.CmsPageKey which results in invalid PageID or you are passing an ID of a page that does not exist in CmsSiteMap provider.
You could get the page ID that is used for the current request using the code below. Then you can query sf_PageBase table to see whether this page exist
1. Create a class that inherits from InternalPage class
InternalPageCustom : InternalPage
// the key will return the PageID you are requesting
var key = (ICmsUrlContext)
2. Replace the default internal page in /Sitefinity/cmsentrypoint.aspx
<%@ Page Inherits=
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