SQL Reporting Services Access Denied Error

When trying to access your Report Manager, you might see an Access Denied error with an entry in your application log like:

Thankfully it's a simple one to fix. It's caused because ASP.NET cannot access the Reporting Services folders. I'm not sure what the minimum access permissions are but this was on dev so I simply gave the Network Service full control on my 'ReportManager' and 'ReportServer' folders and that sorted things.