I thought I would post this as a reference for those having issues with an alias redirect problem after an upgrade:

I was on DNN 4.8.0 with multiple portals and wanted to upgrade to 4.8.4. After the installation (which appeared successful), none of my portals would work. They would initiate an 302 redirect infinite loop to the exact same alias. (i.e. http://www.fakesite.com/Default.aspx?alias=www.fakesite.com) 

After verifying that my PortalAlias table contained the proper aliases - and pulling about half my hair out looking for a solution - I decided to start comparing my back-up 4.8.0 site with the 4.8.4 upgrade. I found that the default.aspx was changed during the upgrade. (I noted that the default.aspx in the upgrade package was NOT the same as the one in my site's root, therefore it must have changed between the time I copied the file and when the actual upgrade was performed.)

The new default.aspx file only contained the Page_Load method with some code that created a redirection URL. After replacing the mysterious default.aspx file with the actual default.aspx that came with the upgrade back into the root, everything was good.

** My questions: Obviously that is by design, by why did it not get replaced by the proper default.aspx after the upgrade was complete? Is that an upgrade bug that should be looked into or did I skip a step in my upgrade procedure? What is the purpose of the alternate default.aspx?

Thanks!
-sirchuy