You would place that web.config file in that website folder whose name you show. And if one is there already, you'd instead carefully fold these changes into that.
Or you could instead use the iis ui. At this web site level, find the "error pages" feature, open that, then use the "edit feature settings" on the right, and change the value to "detailed". (Google for more details if needed.) Now refresh the page.
Either way, you should now see the actual error that is happening because of your password change.
Finally, you should NOT need to be using a personal username and password for accessing the folder in question. Your iis site runs by default under a special windows user account that incorporates the name of the app pool used by the site, and you should be able to grant that user permission to the folder in question. This, too, is something you should be able to learn more about in any of many online resources.
Indeed, as it's not at all a question specific to cf, you don't need to give regard to cf at all in looking into it. That should widen your search results.
Let us know if you solve things, or if you fail to find help. Of course, others here may offer an expansion on what I've offered, or they may well offer an entirely different answer (even one which entirely ignores what I've said, as can happen here sometimes).
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