HTTP Error 404.0 after installing 2sxc v9.33 on DNN 8.00.04

Installed 2sxc for the first time on a site. Immediately had problems with the 2sxc UI telling me 404 page not found. Switched to the server machine and IIS was showing the real error:

I am not able to pin down the real issue. Anyone recognize this and know what the fix might be? Seems like a routing or permissions issue maybe?

IIS 7.5 DNN 8.00.04 patched 2sxc 9.33 (see screenshot for full)

Solution

What you're probably seeing is the full-screen iframe, but that's not loading for whatever reason. In the screenshot it looks like "ui.html" was stripped down to "ui?" so I'm guessing you have a server-side rewrite trying to protect html pages.