One works fine, but the other doesnt - I cant say for sure whether it has ever worked on this particular PC, since it has been a while since Ive had to work on it, but I think it has been working in the recent past.

The requested page cannot be accessed because the related configuration data for the page is invalid.

Cannot read configuration file due to insufficient permissions.

This account needs to be able to enumerate all folders within the website, to read the web.config files that live there.

IIS uses web.config to manage IIS settings, not just ASP.Net settings.

So youll either need to allow access to the app pool identity so it can look for a web.config file, or set the allowSubdirConfig property to false to stop it looking.

Ive specifically re-checked the effective permissions at the application level.

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Iis Cannot Verify Access To Path