IIS - Grant Default App Pool

It's called IIS APPPOOL\DefaultAppPool

CMD command

icacls c:\inetpub\wwwroot /grant "IIS APPPOOL\DefaultAppPool":(OI)(CI)(RX)

Of course to complicated things this users is sometimes called IIS_IUSRS

First of all: you don't have to change anything in your config file. It's OK. The problem is with windows file permissions.
This problems occurs because your application can not access and read web.config file.
Make the file accessible to IIS_IUSRS group. Just right click web.config and click properties, under security tab, add IIS_IUSRS.
So what is this IIS_IUSRS thing?
Your web site is like an exe file. Just like any exe file, it should be started by a user and it runs according to permissions assigned to that user.
When your site is started in IIS, Application Pool of your web site is associated with a user (Network Services, Local System, Etc. ...) (and can be changed in IIS)
So when you say IIS_IUSRS, it means any user (Network Services, Local System, Etc. ...) that your site is running as.
And as @Seph mentioned in comment below: If your computer is on a domain, remember that IIS_IUSRS group is a local group. Also make sure that when you're trying to find this user check the location it should be set to local computer and not a corporate domain.