one of 2 websites pointing to same root folder not working well

One of my client has IIS with 2 web sites running, pointing to same app pool and root folder. Lets say those 2 websites as a.com and b.com, pointing to same root folders. Now that b.com works well, but a.com throws 500 internal server error. Browsing within the server gives me the error:

HTTP Error 500.19 - Internal Server Error

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

Detailed Error Information:

Module IIS Web Core

Notification Unknown

Handler Not yet determined

Error Code 0x80070005

Config Error Cannot read configuration file due to insufficient permissions

Config File \?\C:\Inetpub\vhosts\Server\2\web.config

Physical Path

Logon Method Not yet determined

Logon User Not yet determined

Config Source: -1: 0:

The actual web.config file (under root folder) is somewhere else, and the above config location shown in the error is something else. Can someone guide me how to resolve this issue.? I am not aware of any permissions to this web.config and it was working fine earlier. No such permission changes done recently.

Typically that error is presented when the app pool user has no permissions on the app folder. Since you say both web sites are using the same app pool is kind of weird that one is working and the other isn't. I'd suggest that you inspect a little bit more in the IIS logs and see if you find more useful information.