Interesting. I can tell you that a brand new blank 4.26 C++ project should compile without issues. A blank project has nothing introduced by the developer yet which could cause a problem. So I would suggest you completely blow away your Unreal installation (and probably even the Epic launcher) and start from scratch. Install the Epic launcher, install UE 4.26, create a new C++ project and build it. Also I would take this opportunity to upgrade to Visual Studio 2019. In fact you can probably uninstall older versions of Visual Studio if you don’t need them, to avoid problems with the different installs clobbering each other with bad PATH variables/etc that may have built up over time.
Again, all I can really tell you is a blank 4.26 C++ project should build just fine. I see references to Visual Studio 2015 and 2017 in your output. And you mention that you have a UE 4.19 project. My guess is overtime all these different versions of installed tools have accumulated cruft and are now colliding with each other and causing config issues. Hope that helps.