First of all I first checked the url in web browser that is:
/_layouts/CustomErrorPage/DueDateErrorPage.aspx
It was opening the page that means the page is available but there is something missing in the url only.
So by using the SPUrlUtility class I was able to redirect the page.
The changes I needed to made was:
properties.RedirectUrl = SPUrlUtility.CombineUrl(properties.WebUrl,"/_layouts/CustomErrorPage/DueDateErrorPage.aspx");
It's working perfectly..