VS 2015 Redirect between websites in same solution broken

Hello and thanks in advance.

I have a VS 2015 WebSite solution (ASPX) with 2 website projects within it. Until recently, my redirect from one of the websites to the main page of the other website has worked flawlessly, though I noticed that that link in the redirect had a website name used a long time ago, and no longer have that same file (yet the redirect worked).

Now, the redirect is completely broken. I have tried renaming the redirect text, the pathway, folder structure, and web page itself.

The current path is like: "~/Data/Default.aspx" since the second WebPage is located in the "Data" folder. I get the 404 error, cannot find it.

Also, if I set the second web page to be the starting web page in VS, it loads perfectly fine.

I am wondering if I updated something in VS, in Windows, in IIS, or if something might have gotten screwed up with one of these things.

Any ideas for what I should check and test?