A forced Windows 10 update installed successfully. The update history shows "2017-07 Cumulative Update for Windows 10 Version 1703 for x64-based Systems (KB4025342)" among a total of 45 other updates installed on 7/29/2017.
After updating and attempting to launch my project in the debugger, which has always worked before:
I now get a blank page for every URL. I'm debugging in IIS Express on localhost. View Source yields an absolutely blank page also (this is definitely not the IIS file association problem that I've seen documented).
I did absolutely nothing else (to cause this) besides install Windows 10 (and I wouldn't have if MS didn't force me to). After this I did a clean/rebuild completely from scratch, I then removed the Bin folder, re-installed all the NuGet packages, still exactly the same result, F5 launches the web application in debug mode and launches the page as usual but it is completely blank. It does not execute any code, Page_Load is not called.
Visual Studio 2017 made no difference
I switched to IIS and got a 500.19 error with 0x80020021 error code which this page 500.19 error with IIS7.5 addresses, after unlocking the two subsections of system.web.server that were locked (why were they locked?)
it now returns 404.3 Not Found, with:
-- It is possible that a handler mapping is missing. By default, the static file handler processes all content. (well, not .aspx pages!)
-- The feature you are trying to use may not be installed. (somehow maybe this is true, but how?)
-- The appropriate MIME map is not enabled for the Web site or application.
(Warning: Do not create a MIME map for content that users should not download, such as .ASPX pages or .config files.) (right... there should not be a MIME map entry for .ASPX)
-- If ASP.NET is not installed. (how could this be?)
ASP.NET installation -- it is built in to Windows 10, I installed the .NET developer packs and tried 4.5.2, 4.6.1 and 4.7, No change.
In summary: nothing has helped. With IIS Express, or IIS, same problem. All .aspx urls return a completely blank page and no code in the code-behind .cs files gets executed. Fiddler says 503 (but 503 is not shown in the browser, either Edge or Chrome).
Latest unsuccessful effort: I went back to the previous build (rolled back the latest Windows update) by selecting "Go back to an earlier build" from "Update Settings/Windows Update/Update History" and I still have the problem. So, either I'm really stuck now, or I've opened up a lot of possibilities based on a wrong assumption that it was the update that caused the problem. Something is corrupt? Something changed in my project that I'm not aware of? Corrupt: I ran the system file scanner, sfc /scannow and it found no problems. Back to the drawing board...