CF Admin inaccessible after upgrading to ColdFusion2021

Hi @jeffw43645657 

Can you please paste the URL that you are using to access the CF Admin?

Suggestion:

1.  Open the Command Prompt (CMD) as Administrator;
2.  Type ipconfig /flushdns and press ENTER;
3.  Run the Web Server Configuration Tool, C:\ColdFusion2021\cfusion\runtime\bin\wsconfig.exe;
4.  Open the ColdFusion Administrator in a browser other than Internet Explorer. Say, Chrome or Firefox.

Jeff, you've done a great job laying out details. I'm sure it's a frustrating situation. That said, it may be something simple, or it may be something more esoteric. Let me offer several ideas, from most likely (yet easy to rule out) to more challenging.

• First, you may want to make 100% sure cf2021 is running. That error would usually mean the browser could not reach the server indicated in the url. And I'm assuming you're accessing the admin on the cf machine as localhost, right? 
• I know you said other sites "work" in returning a cf error. Could that be a cached page? Try it from another browser. 
• And you are accessing that site through iis, while the cf Admin is being accessed through cf's built in web server, such as at port 8500, right?
• Even then, note that if cf2016 was running at the time you installed 2021, then the latter would use port 8501 instead. Which are you using now, when it fails?  (FWIW, you didn't technically "update" cf2016. Rather cf2021 is a separate install, with its own service, folder, etc. as you may have noticed or already realize.) 

• You're sure you're using the same url for that admin when it fails as when it worked right after the install, right?

If it's not something like the above, then it seems something's gone amiss. It's not some inherent result of installing cf, as it was working at first install. 

Moving on, when you say the first launch of the cf admin migrated settings from the old one, do beware that at that moment, had you restarted cf, perhaps this error would have happened. Why? Because the import could have brought in changes (especially jvm ones) that may not work well with the new version. But that's just a guess, maybe not the issue. (And I doubt you have the original files from before the migration, as cf does not protect them.)

Some more questions that could help us, in that case:

1. what is the url you're using for the admin?
2. And have you tried watching the browser dev tools (even in the  recently obsoleted IE you have?) What does that show, in its network tab? 
3. Have you considered using a more recent browser? It might show better diagnostics. (Or is that "out of the question" in your shop? And if so, do the overlords realize the MS finally EOLed IE last month? Time to revisit those old rules against "any other browser".) 

Let us know how things go. And if you get to needing this fixed without lots of back and forth here (I'm sure others will chime in with more thoughts), note that I could help via a remote consulting session. I doubt it would take 15 mins to sort this out.

That's not a crass sales pitch or bait and switch: it's simply a lot easier to just follow a series of diagnostic steps together live than to elaborate further on each, and the permutations of what they may show, and what next steps would be for each result. If interested, check out the consulting page at carehart.org.

I do hope I may have offered you a solution among all else above, but I'll be following along here either way to hear how it turns out.