Bruce ... I always go through the upgrade installation procedures correctly (and this is the first error I've encountered since 2002)!
However, I tried turning off and then back on again ASP Rewrite rules on my hosted server CP. Oddly enough this seems to have done the trick!! And, I had also deleted the web.config file in the forum root directory.
Strange things seem to happen to these machines at times

. Scotty, it wasn't any of your mods that caused this as I did a totally virgin intall/upgrade and that did not work. Even after totally cleaning the forum directory and uploading the 9.70 I was suffering the dreaded 500 error.
I only implement all my mod changes etc. to the forum on each upgrade
after I know it's working correctly from a vanilla install. Incidentally, your mods are great (which I've now purchased) and have never caused any previous hardships/error!!
So ... now I'm a happy bunny with my forum running as smoothly as it has for the past 8.5 years on Bruce's superb ASP based software!