So, you’ve been denied! It’s OK. It happens to the best of us.
If you are lucky enough be gifted with this message take a look at the NTFS rights of the SQL Server Reporting Services instance which will likely be a folder like C:\Program Files\Microsoft SQL Server\MSRS10_50.MSSQLSERVER\Reporting Services. Granting the appropriate AD group read & execute rights may solve the problem.
Server Error in '/Reports' Application.
Access is denied.
Description: An error occurred while accessing the resources required to serve this request. You might not have permission to view the requested resources.
Error message 401.3: You do not have permission to view this directory or page using the credentials you supplied (access denied due to Access Control Lists). Ask the Web server's administrator to give you access.
Thanks to Mike and Jerry for pointing me in the right direction. After carefully reading the error, it is quite obvious isn’t it.